BEILSTEIN JOURNAL OF NANOTECHNOLOGY
Organic and inorganic-organic thin film structures by
molecular layer deposition: A review
Pia Sundberg and Maarit Karppinen*§
Review
Address:
Department of Chemistry, Aalto University, P.O. Box 16100 FI-00076
Aalto, Finland
Email:
Maarit Karppinen* - maarit.karppinen@aalto.fi
* Corresponding author
§ FAX: +358 9 462 373
Keywords:
atomic layer deposition (ALD); hybrid inorganic-organic thin films;
molecular layer deposition (MLD); nanolaminates; nanostructuring;
organic thin films; superlattices; thin-film technology
Beilstein J. Nanotechnol. 2014, 5, 1104-1136.
doi:10.3762/bjnano.5.123
Received: 20 February 2014
Accepted: 20 June 2014
Published: 22 July 2014
Associate Editor: A. Golzhauser
© 2014 Sundberg and Karppinen; licensee Beilstein-lnstitut.
License and terms: see end of document.
Abstract
The possibility to deposit purely organic and hybrid inorganic-organic materials in a way parallel to the state-of-the-art gas-phase
deposition method of inorganic thin films, i.e., atomic layer deposition (ALD), is currently experiencing a strongly growing interest.
Like ALD in case of the inorganics, the emerging molecular layer deposition (MLD) technique for organic constituents can be
employed to fabricate high-quality thin films and coatings with thickness and composition control on the molecular scale, even on
complex three-dimensional structures. Moreover, by combining the two techniques, ALD and MLD, fundamentally new types of
inorganic-organic hybrid materials can be produced. In this review article, we first describe the basic concepts regarding the MLD
and ALD/MLD processes, followed by a comprehensive review of the various precursors and precursor pairs so far employed in
these processes. Finally, we discuss the first proof-of-concept experiments in which the newly developed MLD and ALD/MLD
processes are exploited to fabricate novel multilayer and nanostructure architectures by combining different inorganic, organic and
hybrid material layers into on-demand designed mixtures, superlattices and nanolaminates, and employing new innovative
nanotemplates or post-deposition treatments to, e.g., selectively decompose parts of the structure. Such layer-engineered and/or
nanostructured hybrid materials with exciting combinations of functional properties hold great promise for high-end technological
applications.
Introduction
Many high-end technologies rely on our capability to fabricate
thin films and coatings with on-demand tailored compositions
and architectures in a highly controlled way. The atomic layer
deposition (ALD) technique is capable of producing high-
quality nanometer-scale thin films in an atomic layer-by-layer
manner. Compared with other advanced gas-phase thin-film
deposition techniques, ALD has several distinct advantages:
The films can be deposited with a great control over the film
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Beilstein J. Nanotechnol. 2014, 5, 1104-1136.
thickness and they are not only pinhole free, dense and uniform,
but also conformal even when deposited on complex three-
dimensional (3D) structures. These features make ALD a
method of choice for nanotechnology, for both material syn-
thesis and device fabrication. The technology spectrum in
which ALD can be utilized is extremely wide, including micro-
electronics, catalysis, energy applications and protective/barrier
coatings.
The history of ALD goes back to the 1960s and 1970s [1-4].
Traditionally, ALD has been used to fabricate rather simple
well-known inorganic materials, such as binary oxides and
nitrides. The range of materials was fundamentally broadened
by experiments producing organic polymers in the 1990s by a
variant of ALD, now commonly known as molecular layer
deposition (MLD), named after the molecular layer-by-layer
fashion the film grows during the deposition [5-9], Then - most
excitingly - in the late 2000s the two techniques, ALD and
MLD, were combined to produce inorganic-organic hybrid ma-
terials (Figure 1), making it possible to synthesize totally new
material families with versatile characteristics, which are not
accessible by any other existing technique [10-14].
■ organic
■ hybrid
II 1 1 1 1 1 1 1 li
1
1990 1995 2000 2005 2010
Year
Figure 1 : Number of articles annually published featuring organic and
hybrid inorganic-organic thin films deposited by MLD and ALD/MLD.
In the combined ALD/MLD process organic molecules are
covalently bonded to the metal atoms and vice versa, forming
periodic thin-film structures that can be imagined to consist of
either interlinked hybrid inorganic-organic polymer chains of
essentially identical lengths or alternating two-dimensional (2D)
planes of inorganic and organic monolayers (Figure 2). The
hybrid thin films may not only possess properties combined
from those of the two parent materials, but may also have
completely new material properties, making them excellent
candidates for a wide range of applications. Possible uses for
the hybrid ALD/MLD films include optoelectronic devices,
sensors, flexible electronics, solar cell applications, and protec-
tive coatings, to name only a few. It is also straightforward to
make porous structures from the ALD/MLD grown hybrids by
removing the organic part by simple annealing or wet-etching
procedures [15,16]. Further tuning of material properties may
be achieved by combining different inorganic, organic and
hybrid layers into various thin-film mixtures, superstructures
and nanolaminates. For example, precise control of the refrac-
tive index is extremely important in optical applications [17],
while control of the electrical properties is required for storage
capacitors, non-volatile memories as well as for transparent
thin- film transistors [18,19]. Moreover, the tunability of the
surface roughness is advantageous when fabricating gas sensors
[20].
Figure 2: Schematic illustration of purely organic thin films grown by
MLD (left) and hybrid inorganic-organic thin films grown by ALD/MLD
(right).
Over the years a number of excellent reviews featuring various
types of ALD processes have been published, most recently,
e.g., by Puurunen [4], George [21] and Miikkulainen et al. [22].
A review by Knez et al. [23] focuses on nanostructure fabrica-
tion by ALD. Although the introduction of the MLD method
dates back two decades, the number of articles featuring purely
organic thin films is still quite limited. Nevertheless some
reviews concerning MLD-based thin films have been published
in the past: George et al. [24] discuss the surface chemistry of
MLD grown materials, addressing the problems which arise
when using organic precursors in the growth process; Leskela et
al. [25] shortly review the novel materials fabricated by ALD
and MLD; George [26], George et al. [27] and Lee et al. [28]
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focus on metal alkoxide thin films; Yoshimura et al. [29]
discuss a possibility to utilize MLD in cancer therapy applica-
tions; King et al. [30] describe fine particle functionalization by
ALD and MLD; and the review by Zhou et al. [31] covers all
the organic interfaces fabricated by MLD.
The aim of this review is to provide a thorough investigation of
the various thin films deposited by taking advantage of the
currently strongly emerging MLD technique, including pure
organic thin films, hybrid inorganic-organic thin films and their
mixtures and nanolaminate structures. First we will describe the
sequential ALD/MLD process, followed by a few words about
the organic precursors used in these processes. Then all the
various materials fabricated utilizing MLD are reviewed: The
purely organic materials are summarized first and the inor-
ganic-organic materials are discussed in a separate chapter.
Lastly, the variously mixed and nanostructured ALD/MLD and
MLD materials are presented.
Review
Deposition cycle and ideal ALD/MLD growth
In both ALD and MLD the gas-solid reactions occur in a self-
limiting, surface-saturated manner. The characteristic ALD/
MLD growth can be described by a so-called ALD and/or MLD
cycle. The number of precursors employed during an ALD or
MLD process can be varied, but a prototype process is based on
two. For example, in case of the hybrid inorganic-organic films
one inorganic and one organic precursor are used, and the ALD/
MLD cycle can be separated into four steps consisting of
precursor pulsing and intermediate purging steps as described in
Figure 3.
To ensure the self-limiting growth both the precursor pulsing
and purging steps should be sufficiently long. In an ideal
process the surface is fully covered with the precursor in each
precursor pulsing step, but in practice only a partial coverage is
typically achieved. The so-called growth-per-cycle (GPC) value
is the average increase in film thickness during one ALD/MLD
cycle.
When the GPC remains constant with increasing number of
deposition cycles, the growth is said to be linear. In some cases,
however, the GPC is not constant from the beginning. The sub-
strate may inhibit or enhance the film growth depending on the
compatibility of chemistries of the substrate surface and the
growing film, in which case the GPC is initially lower or higher
before settling to a constant value [4]. It is the sequential self-
limiting nature of ALD and MLD that enables the great thick-
ness control and conformal growth of the films, which in turn
makes the two techniques, ALD and MLD, and their combina-
tions such a great asset for nanotechnology.
Figure 3: An ALD/MLD cycle consisting of the following four steps:
(1 ) the first (inorganic) precursor is pulsed to the reactor and it reacts
with the surface species, (2) the excess precursor and possible
byproducts are removed from the reactor, either by purging with inert
gas such as nitrogen or argon, or by evacuation, (3) the second
(organic) precursor is pulsed to the reactor and it reacts with the
surface species, and finally (4) the excess precursor/possible byprod-
ucts are removed from the reactor. In an ideal case a monolayer of a
hybrid inorganic-organic material is formed. To deposit thicker films
this basic ALD/MLD cycle is repeated as many times as needed to
reach the targeted film thickness.
The ALD or MLD growth typically depends on the deposition
temperature at least in some temperature ranges. The effect of
temperature on the GPC value is often described by a concept
known as an ALD (or MLD) window (Figure 4a). The ALD
window has been defined as a regime in which the GPC
remains constant and does not depend on process parameters
like temperature, gas pressure, precursor flows or purging
times. Outside the ALD window the GPC value may be higher
due to precursor condensation (at too low deposition tempera-
tures) or decomposition (at too high deposition temperatures),
whereas limited growth may result from insufficient reactivity
(at too low deposition temperatures) or desorption (at too high
deposition temperatures) of the precursor. However, the exis-
tence of an ALD window is not a necessary prerequisite for an
ALD-type growth, and such a window is not found for all well-
behaving ALD (or MLD) processes. Examples of typical cases
in which no temperature range of constant growth is seen, but
the process may yet be highly reproducible are shown in
Figure 4b-d. The growth may occur in the way shown in
Figure 4b when the growth is not fully of ALD type, but one of
the precursors diffuses into the film, improving the growth by
providing more reactive sites: The diffusion out of the film is
enhanced at higher temperatures, resulting in a lower growth
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Figure 4: Dependence of the film growth on the deposition temperature: (a) within the so-called ALD window the growth per cycle remains constant
with increasing temperature, whereas (b)-(d) represent typical cases in which no temperature range of constant growth is seen, but the process may
yet be highly reproducible.
rate [12]. The decrease in growth at increasing deposition
temperatures may be observed in general when the temperature
affects the number of reactive sites or the reaction mechanism.
The combined effect of reaction activation (increase in growth),
followed by decrease on reactive sites may result in a growth
such as shown in Figure 4d [4].
The MLD and ALD/MLD films often show lower than antici-
pated GPC values. There are several possible causes for the
M T I ?
Figure 5: Ideally, the organic precursor molecule reacts with one
surface site only and remains straight (left). It may also react twice with
the surface (middle) or tilt (right).
hindered growth (Figure 5). Organic precursor molecules with
long chains are likely to tilt such that the growth is not perfectly
perpendicular to the surface. Likewise, organic molecules may
bend and react twice with the surface, reducing the number of
reactive surface sites and lowering the growth rate. Organic
precursors are also often bulky, causing steric hindrance. The
various difficulties encountered when using organic precursors
are discussed in detail in a review by George et al. [24]. Several
strategies have been employed to improve the controllability of
the growth process, such as using organic precursors with stiff
backbones [13,14,32-37] or with two different functional
groups [36-39], using reactions requiring surface activation
[10,39-46], using precursors in which ring-opening reactions
occur [47], or using three different precursors instead of two
[48,49].
The ratio, r = GPCIML, where ML is the ideal length of the
M-R monomer (without bending, see Figure 5), provides us
with a measure of the perfectness of the growth. Thus, ideally
r = 1. The r value achieved varies greatly with different
precursor combinations. For purely organic MLD films r is
typically lower than 0.5, exceptions being the hexanedioyl
dichloride+hexane-l,6-diamine [8,50] and heptane- 1,7-di-
amine+nonanedioyl dichloride [7] systems. For hybrid ALD/
MLD thin films there is a larger variation in the r values
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depending on the organic precursor employed. The choice of
the metal precursor seems to have a significant effect, too. For
example, with the linear ethane- 1,2-diol (ethylene glycol, EG)
molecule as the organic precursor and trimethylaluminum
(A1(CH 3 ) 3 , TMA), diethylzinc (Zn(C 2 H 5 ) 2 , DEZ), titanium
tetrachloride (TiC^) and zirconium ?er?-butoxide
(Zr(CH4H90)4, ZTB) as the inorganic precursors the growth
processes have yielded r values of 0.6 0.1, 0.6 and 0.2, respect-
ively [12,51-54]. In case of the aromatic precursor benzene- 1,4-
diol (hydroquinone, HQ), r values of 0.4 and 0.2 have been
achieved for Al-based [13] and Zn-based [33] films, respective-
ly. Systems exhibiting r values close to unity have been
reported, such as TiCl4+4-aminophenol [37] and hexa-2,4-
diyne-l,6-diol (HDD) [55,56] containing hybrid films. In case
of the TiCl4+4-aminophenol process, the excellent growth
could be attributed to the two different functional groups and
the stiff aromatic backbone of 4-aminophenol as well as to the
small -CI ligands in TiCl4 [37]. The HDD molecule with two
triple bonds is also stiff and during the deposition process also
the formation of bridging alkanes is induced by UV radiation
after precursor pulsing steps [55,56]. However, as the hybrid
systems seem to be sensitive regarding the process parameters,
especially considering pulsing and purging times, there are
systems with r values which are considerably higher than 1,
e.g., TMA+heptanedioic acid [57] and TMA+oxiran-2-
ylmethanol (glycidol, GLY) [38].
The characterization techniques used to investigate the thin
films deposited by using MLD do not vary much from those
techniques used for inorganic thin films grown by ALD. An in
situ quartz crystal microbalance (QCM) is often used to give
some insight on the growth dynamics of the deposition. Besides
thickness measurements, X-ray reflectivity (XRR) can be used
for evaluating densities and roughnesses of the thin films. The
crystallinity of the films is examined by X-ray diffraction
(XRD). The topography of the films can be investigated by
using atomic force microscopy (AFM). Fourier transform
infrared (FTIR) spectroscopy is useful for analyzing the chem-
ical state of the films. The composition of the films can be
studied by X-ray photoelectron spectroscopy (XPS), whereas
the presence of a metal can be verified by X-ray fluorescence
(XRF) measurements. Nanoindentation gives insight on the
mechanical properties of the films.
Organic precursors employed in MLD
Both ALD and MLD set some requirements for the precursors
employed, such as sufficient vapor pressure, reactivity and
stability at the reaction temperature, to ensure feasible film
growth. Finding organic compounds which would fulfill these
requirements is not straightforward. Many of the organic
precursors exhibit low vapor pressures at room temperature and
it is thus mandatory to heat them to achieve a sufficient
precursor supply. In Table 1, we list all the organic compounds
employed/investigated as precursors for ALD/MLD. Here it
should be noted that organic compounds typically have several
different names; the nomenclature we use is based on the
recommendations of the International Union of Pure and
Applied Chemistry (IUPAC), but in Table 1 commonly used
other names for the compounds are also given. It should also be
emphasized that not all the processes based on the precursors
listed in Table 1 exhibit the characteristic features of an ALD/
MLD process. In Table 1 we give - when accessible - the vapor
Table 1 : Organic compounds (and their different names and abbreviations) employed in MLD and ALD/MLD processes together with vapor pres-
sures, P, at 100 °C and temperatures, T, corresponding to a vapor pressure of 2 mbar for some of the organic precursors (the values were calculated
IUPAC name
abbreviation names used in references
P(Pa) 7" CO
(1£)-prop-1-ene-1,2,3-tricarboxylic acid
(2£,4E)-hexa-2,4-dienedioic acid
(2S)-2-aminopentanedioic acid
(E)-butenedioic acid
(Z)-butenedioic acid
1,2-bis[(diamethylamino)dimethylsilyl]ethane
1 ,4-diaminobenzene
1 ,4-diisocyanatobenzene PDIC
1 ,4-diisocyanatobutane
1 ,4-diisothiocyanatobenzene
2-aminoethanol
2-oxepanone
2,2'-(propane-2,2-diylbis(oxy))-diethanamine
frans-aconitic acid
(2E,4E)-hexa-2,4-dienedioic acid;
frans.frans-muconic acid
L-glutamic acid
fumaric acid; (E)-butenedioic acid
maleic acid; (Z)-butenedioic acid
1,2-bis[(diamethylamino)dimethylsilyl]ethane
p-phenylenediamine; 1 ,4-phenylenediamine
1,4-phenylene diisocyanate
1 ,4-diisocyanatobutane
1,4-phenylene diisothiocyanate
ethanolamine
e-caprolactone
6.37
154
6410
1520
182
165
142
104
43
58
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Table 1 : Organic compounds (and their different names and abbreviations) employed in MLD and ALD/MLD processes together with vapor pres-
sures, P, at 100 °C and temperatures, T, corresponding to a vapor pressure of 2 mbar for some of the organic precursors (the values were calculated
om the DIPPF
! Project 801 database (full version) [58]). (continued)
4,4'-oxydianiline
ODA
4,4'-oxydianiline; 4,4-diaminodiphenyl ether
4-aminophenol
AP
4-aminophenol
4-nitrobenzene-1 ,3-diamine
2,4-diaminonitrobenzene
7-octenyltrichlorosilane
7-OTS
7-octenyltrichlorosilane
8-quinolinol
8-hydroxyquinoline
199
100
benzene-1 ,2,4,5-tetracarboxylic acid
1 ,2,4,5-benzene tetracarboxylic acid;
1 ,2,4,5-benzotetracarboxylic acid
283
benzene-1 ,2-dicarboxylic acid
1 ,2-benzenedicarboxylic acid;
1 9-hp-n7piHipnrhfiY\/lip apiH
173
benzene-1 ,3,5-tricarboxylic acid
1 ,3,5-benzene tricarboxylic,
1 ,3,5-benzotricarboxylic acid
benzene-1, 3, 5-triol
1 ,3,5-benzenetriol; phloroglucinol
Del \Z.C\ lc- I ,0-UIOdl UUXyilO dOlU
1 ,3-benzene dicarboxylic acid;
1 ,3-benzodicarboxylic acid
£.00
benzene-1 ,4-dicarboxylic acid
1 ,4-benzene dicarboxylic acid;
1 ,4-benzodicarboxylic acid
266
benzene-1 ,4-diol
1 ,4-benzendiol; hydroquinone
9.39
137
benzoic acid
benzoic acid
1820
71
but-2-yne-1 ,4-diol
2-butyne-1 ,4-diol
•1 -1 Q
1 To
-i n7
butane-1 ,4-diamine
1 ,4-butane diamine
butanedioic acid
succinic acid
0.569
168
decane-1,10-diamine
1,10-diaminodecane
decanedioic acid
decanedioic acid; sebacic acid
0.0466
198
decanedioyl dichloride
sebacoyl dichloride
ethane-1,2-diamine
ED
ethylenediamine
56700
ethane-1,2-diol
EG
ethylene glycol
2100
61
ethanedihydrazide
oxalic dihydrazide
ethanedioic acid
ethanedioic acid; oxalic acid
50.9
117
ethanetetracarbonitrile
tetracyanoethylene
furan-2,5-dione
maleic anhydride
3260
48
fnmT^ 4-fir91hpn7nfi ir^n-1 *3 *-! 7-tp-trnnp
I Ul U[0, t + I J L^JUCI 1Z.U IUIdM I,J,J,f iGll (J I lc
PMDA
pyromellitic dianhydride;
1 ,2,3,5-benzenetetracarboxylic anhydride
heptane-1 ,7-diamine
1,7-diaminoheptane
heptanedioic acid
heptanedioic acid; pimelic acid
0.806
175
hexa-2,4-diyne-1 ,6-diol
HDD
2,4-hexadiyne-1,6-diol; hexadiyne diol
hexane-1,6-diamine
1 ,6-hexanediamine; 1 ,6-diaminohexane;
3670
43
hexamethylene diamine
hexanedioyl dichloride
adipyl dichloride; adipoyl chloride
A/-(2-aminoethyl)ethane-1 ,2-diamine
diethylenetriamine
2610
52
A/,A/-bis(2-aminoethyl)ethane-1 ,2-diamine
triethylenetetramine
62.5
117
nonanedioyl dichloride
azelaoyl dichloride
octane-1,8-diamine
1,8-diamino-octane
octanedioic acid
octanedioic acid; suberic acid
0.109
185
octanedioyl dichloride
suberoyl dichloride
oxiran-2-ylmethanol
GLY
glycidol
11400
27
pentanedioic acid
pentanedioic acid; glutaric acid
3.26
160
propane-1 ,2,3-tricarboxylic acid
tricarballylic acid
propane-1,2,3-triol
GL
glycerol
25.6
131
propanedioic acid
propanedioic acid; malonic acid
4.55
147
propanedioyl dichloride
malonyl chloride
terephthalaldehyde
terephthalaldehyde
320
94
terephthalic acid bis(2-hydroxyethyl) ester
terephthalic acid bis(2-hydroxyethyl) ester
0.00091
243
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Table 1 : Organic compounds (and their different names and abbreviations) employed in MLD and ALD/MLD processes together with vapor pres-
sures, P, at 100 °C and temperatures, T, corresponding to a vapor pressure of 2 mbar for some of the organic precursors (the values were calculated
3 ro
terephthaloyl dichloride
tris(2-aminoethyl)amine
tris(2-hydroxyethyl)amine
terephthaloyl dichloride; terephthaloyl chloride 172 103
tris(2-aminoethyl)amine
triethanolamine 1.90 168
pressures at 100 °C together with the temperatures corres-
ponding to a vapor pressure of 2 mbar, which is a rather typical
pressure used for ALD/MLD depositions.
Organic thin films
So far MLD has been used to produce polyamide, polyimide,
polyimide-amide, polyurea, polyurethane, polythiurea, poly-
ester and polyimine thin films. In Table 2, we list the character-
istic polymer linkages seen in these films. For example,
polyamides are polymers in which the precursors employed are
combined with each other via amide bond formation whereas
polyureas contain the urea linkage. And like the name of poly-
imide-amides suggests, these polymers contain both an imide
and an amide group. In the following sub-chapters all the
different types of polymer thin films deposited by MLD until
now are shortly presented.
Polyamides
Polyamides are extremely durable and strong, making them
useful materials for a wide variety of applications, e.g., textiles,
automotive industry applications and electronics. Polyamides
can be classified to be aliphatic, semi-aromatic or aromatic,
depending on the composition of the polymer chain. The
polyamides produced by MLD have been deposited by using di-
amines and acyl dichlorides as precursors (Table 3). The
majority of these polyamides are aliphatic; so far only one semi-
aromatic and one aromatic polyamide have been fabricated.
Table 2: Characteristic linkages for the polymer types deposited using
MLD.
polymer
polyamide
polyimide
polyurea
polyurethane
polythiourea
polyester
polyimine
characteristic linkage
R"
O O
R^N^R
R' R'"
r JV r '
R"
%-V r "
R R'"
O
R^R
Aliphatic polyamides, i.e., nylons, have been deposited by
MLD from precursors with a wide range of chain lengths. The
shortest monomers employed are hexanedioyl dichloride and
hexane-l,6-diamine which have been used to make nylon 66
[8,50]. In the earlier work done by Shao et al. [8] the films were
grown through polycondensation. The in situ FTIR studies by
Du et al. [50] indicate that the deposition of nylon 66 displays
characteristic ALD-type growth, i.e., linearity and self-limiting
growth with the different precursor exposure lengths, although
some ammonium chloride salt formation was observed. The
highest GPC values for nylon 66 were 13.1 A per cycle when
deposited at 60 °C on pretreated Si(100) [8] and up to 19 A per
cycle on KBr substrates at the deposition temperature of 83 °C
[50]. The latter value is somewhat higher than what the
predicted unit-chain length, 17.4 A, would suggest: This higher
than predicted growth rate was attributed to a CVD-type
growth. Kubono et al. [6] deposited nylon 79 from heptane- 1,7-
diamine and nonanedioyl dichloride at room temperature. The
GPC value achieved, 18 A per cycle, was quite close to the
calculated length of the repeating unit of the polyamide, i.e.,
22 A. Nagai et al. [7] fabricated several series of different
nylons, including systems where up to four precursors were
used, but the growth rates of the films were not discussed.
Peng et al. [59] used butane- 1 ,4-diamine and terephthaloyl
dichloride to grow semi-aromatic polyamide thin films. The
highest achieved growth rate of this type of polyamide on
Si(100) substrates was only 2 A per cycle at 85 °C. The near-
edge X-ray absorption fine structure spectroscopy measure-
ments showed that the oligomer units of the films were not
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precursor B
references
hexanedioyl dichloride
O
cr - ' - " Cl
nonanedioyl dichloride
Ck /\ /\ /\ XI
hexane-1,6-diamine
H 2 NL
"NH 2
heptane-1 ,7-diamine
H 2 N N H 2
[8,50]
[6]
hexanedioyl dichloride
O
CI
CI
octanedioyl dichloride
O
decanedioyl dichloride
hexane-1,6-diamine
H,N
octane-1,8-diamine
decane-1,10-diamine
[7]
terephthaloyl dichloride
C> /=N CI
terephthaloyl dichloride
O /=\ CI
butane-1,4-diamine
NH 2
H 2 N
1 ,4-diaminobenzene
[59]
[60]
perpendicular to the substrate surface but significantly tilted,
suggesting that double reactions take place during the growth.
The only aromatic polyamide grown by MLD so far was fabri-
cated by using terephthaloyl dichloride and 1,4-diaminoben-
zene as precursors. In industry, these two chemicals are used as
precursors for a mechanically strong and thermally stable
polymer known as Kevlar. The growth rate for the tere-
phthaloyl dichloride+l,4-diaminobenzene MLD process was
not constant: At 145 °C the measured GPC varied between 0.5
and 3.3 A per cycle, which is considerably less than what could
be assumed from the calculated chain length, which is 12.9 A.
The low growth rate was assumed to be due to the non-ideal
polymer-chain orientation: rather than standing up the polymer
chains were suggested to have a more parallel orientation
towards the substrate. [60]
Polyimides
As with the polyamides discussed above, polyimides can be
classified to be aliphatic, semi-aromatic or aromatic depending
on the chain composition. Furo[3,4-/][2]benzofuran- 1,3,5,7-
tetrone (pyromellitic dianhydride, PMDA), widely used as a
raw material for polyimides, has also been used as a precursor
in all the MLD works on polyimide thin films so far (Table 4).
Putkonen et al. [61] combined PMDA together with ethane-1,2-
diamine (ED) and hexane-l,6-diamine to produce semi-
aromatic polyimides. The highest growth rates achieved for
these thin films were 3.9 A per cycle for the former and 5.8 A
per cycle for the latter case, both at the deposition temperature
of 160 °C. As the chain lengths for the ED and hexane-l,6-di-
amine containing polyimide units are 9.8 and 14.5 A, respect-
ively, the GPC values achieved are quite far from those
1111
BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
precursor A precursor B precursor C references
furo[3,4-f][2]benzofuran-1 ,3,5,7-tetrone ethane-1 ,2-diamine
O O
[61]
O O
furo[3,4-f][2]benzofuran-1 ,3,5,7-tetrone hexane-1 ,6-diamine
O O
[61,62]
O O
furo[3,4-/][2]benzofuran-1 ,3,5,7-tetrone 4-nitrobenzene-1 ,3-diamine
O O NH 2
[5]
O O N0 2
furo[3,4-/][2]benzofuran-1 ,3,5,7-tetrone 4,4'-oxydianiline
[5,61,63-66]
O O
furo[3,4-/][2]benzofuran-1 ,3,5,7-tetrone 1 ,4-diaminobenzene
O O
[61,67]
O O
furan-2,5-dione 1 ,4-diaminobenzene furo[3,4-/][2]benzofuran-1 ,3,5,7-tetrone
O O
O^V^O H 2 NH^NH 2
o o
expected for an ideal MLD growth [61]. In a later study Salmi
et al. [62] also grew polyimide films from PMDA and hexane-
1,6-diamine. The depositions were done at the temperature of
170 °C and a GPC of 5.6 A per cycle was reported. The article
concentrates on nanolaminates fabricated from Ta20s and the
polyimide and will be discussed in more detail later in this
review [62].
In all the other polyimides deposited by MLD the second
precursor used has been aromatic. When 4-nitrobenzene-l,3-di-
amine was employed together with PMDA, no strong bonds
formed between the two species, preventing the successful film
growth [5].
The aromatic diamine precursor 4,4'-oxydianiline (ODA) has
been used by several research groups. The highest growth rates
reported were obtained by Yoshimura et al. [5]: In their experi-
ments a GPC value as high as 6.7 A per cycle on an unspeci-
fied substrate was achieved. In later experiments carried out by
Putkonen et al. [61] the highest GPC value obtained was 4.9 A
per cycle on Si(100) and soda lime glass substrates at 160 °C.
Yoshidaet al. [63,64] employed Au-coated Si substrates, modi-
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BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
fied with 4-aminothiophenol to obtain NH2-group terminated
surfaces. They conducted the experiments at 170 °C, achieving
a GPC value of 2.0 A per cycle. As the approximate length of
the PMDA-ODA chain is 14.9 A, none of the groups achieved
GPC values close to full monolayer coverage. Yoshida et al.
[63] also modified the films electrically by using a scanning
probe microscope and reported significant increases in the
conductivity of the films. The precursor combination,
PMDA+ODA, was also employed by Haq et al. [65], who used
reflection-absorption infrared spectroscopy to study the growth
on Cu(l 10) surfaces as a function of temperature and coverage
but did not discuss growth rates of the films. Also Miyamae et
al. [66] deposited thin films by using PMDA and ODA, but as
PMDA was introduced to the reactor so that the pulsing of
ODA was kept on, the growth process was not necessarily of
the real MLD type.
The combination, PMDA+l,4-diaminobenzene, was used to
fabricate organic thin films by Putkonen et al. [61], and also by
Bitzer and Richardson [67]. The former group achieved the
highest GPC value of 1 .4 A per cycle, which is well below the
calculated chain length of 14 A, when using Si(100) and soda
lime glass substrates at the deposition temperature of 160 °C.
The latter group fabricated ultrathin films on Si(100)-2xl at
room temperature but no GPC value was reported. Bitzer and
Richardson [68] later also deposited ultrathin films on Si(100)-
2x1 first functionalized with maleic anhydride, followed by
stepwise exposures of 1 ,4-diaminobenzene and PMDA at room
temperature.
Polyimide-amides
Miyamae et al. [66] deposited polyimide-amide thin films by
using PMDA and terephthaloyl dichloride as the first and third
building blocks, whereas ODA or decane-l,10-diamine were
used as the second one (Table 5). However, as with the
PMDA-ODA films grown by the same group, also with the
polyimide-amides the precursor pulses were overlapping and
thus the process was not precisely of the MLD type.
Polyureas
Polyureas are tough elastomers with a high melting point. They
are especially useful as protective coatings. The polymers form
with a reaction between an isocyanate and an amine. Depending
on the diisocyanate used to fabricate the polymer, polyureas can
be divided to either aromatic or aliphatic systems. The aromatic
polyureas are typically sensitive to light, changing color after
exposure, whereas the aliphatic polyureas retain their color
when treated similarly. Both types have been deposited by using
MLD (Table 6).
1,4-diisocyanatobenzene (1,4-phenylene diisocyanate, PDIC) is
the only aromatic diisocyanate so far used to fabricate polyureas
by MLD. Kim et al. [69] employed ED as the second precursor,
depositing the films on Ge(100)-2xl at room temperature. The
growth of the ultrathin films was investigated with multiple-
internal-reflection Fourier-transform infrared spectroscopy,
demonstrating the formation of urea linkages, although some
imperfections in the growth, possibly due to double reactions of
ED, were also observed. In a later study by Loscutoff et al. [70]
the same precursors were used to fabricate thin films on
Si(100), treated with 3-aminopropyltriethoxysilane before depo-
sitions to get amine-terminated surfaces. The deposition
temperatures used ranged from 25 to 100 °C, and the GPC of
the films decreased from 4.1 A per cycle at 25 °C to 0.4 A per
cycle at 100 °C. A full monolayer coverage was not achieved as
the highest GPC value was well below the PDIC+ED molecule
length of 12.6 A, and also below the somewhat lower GPC
value of 6.5 A per cycle expected based on the density func-
Table 5: Precursors used to deposit polyimide-amide thin films.
precursor A
precursor B
precursor C
reference
furo[3,4-/][2]benzofuran-1,3,5,7-tetrone
4,4'-oxydianiline
terephthaloyl dichloride
O
O /=\ CI
H 2 N^^O^^NH 2
[66]
0
0
furo[3,4-f][2]benzofuran-1,3,5,7-tetrone
decane-1 , 1 0-diamine
terephthaloyl dichloride
0
O /=\ CI
[66]
O
0
1113
BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
precursor A
precursor B
references
1 ,4-diisocyanatobenzene
0=C=NH^y)-N=C=0
1 ,4-diisocyanatobenzene
Q=C=N^ ^N=C=Q
1 ,4-diisocyanatobutane
O^C^N— -^ N=C = 0
1 ,4-diisocyanatobutane
0=C = N^— N=C = °
1 ,4-diisocyanatobutane
0=C = N^-- N=C = 0
1 ,4-diisocyanatobutane
0=C=N'
,N=C=0
ethane-1,2-diamine
H 2 N V
NH P
2,2'-(propane-2,2-diylbis(oxy))-diethanamine
H 2 N^\ o y o ^^NH 2
ethane-1,2-diamine
H * N ^NH 2
N-(2-aminoethyl)ethane-1,2-diamine
H 2 N
"HH 7
A/,W-bis(2-aminoethyl)ethane-1,2-diamine
,NH 2
H 2 N
H
-N
H
tris(2-aminoethyl)amine
H 2 N^^ N /^NH 2
NH 2
[69-71]
[72]
[73]
[73]
[73]
[73]
tional theory calculations carried out by the group. The film
growth was observed to be linear. The films were stable in
ambient air and when annealed at least up to 250 °C [70].
Recently, Prasittichai et al. [71] achieved a GPC of 5.3 A per
cycle when growing PDIC+ED films at room temperature. In
their study self-assembled monolayers fabricated by using
octadecyltrichlorosilane were used to prevent the growth of the
PDIC+ED polymer, enabling the growth of patterned 3D struc-
tures [71].
Zhou et al. [72] used 2,2'-(propane-2,2-diylbis(oxy))-
diethanamine together with PDIC, and fabricated the films on
Si(100) at room temperature. The growth rate of these films was
ca. 6.5 A per cycle, which is considerably less than the chain
length of the expected molecule (18 A). The study demon-
strated that when treated with acid, the backbone of the formed
film reacted from the acid-labile groups. When exposed to basic
solution, the polymer films were stable. These experiments
proved that MLD can be utilized to fabricate photoresist ma-
terials. To make the film a photoresist material, a treatment with
triphenylsulfonium triflate, a photoacid generator, was required
after the deposition.
ethyl)ethane-l,2-diamine and tris(2-aminoethyl)amine have
been used as the second precursor, yielding cross-linked
polyurea films with GPCs of 6.3, 6.7, 3.2, and 3.1 A per cycle,
respectively. The depositions were carried out at room tempera-
ture and Si(100) was used as the substrate. As the calculated
chain lengths for the respective systems are 13.5, 17.2, 20.9,
and 17.2 A, the growth rates of the polymers were considerably
less than a full monolayer per cycle [73].
Polyurethanes
As with polyureas, the reaction in which polyurethane forma-
tion takes place involves an isocyanate, but instead of an amine,
an alcohol is required for the urethane linkage. In the MLD-
grown polyurethanes the isocyanate has been PDIC, whereas
but-2-yne-l,4-diol and terephthalic acid bis(2-hydroxyethyl)
ester has been used as the second precursor in experiments done
by Lee et al. [74] (Table 7). As the aim of the group was to
make templates for the synthesis of zeolites, the growth process
of the films is not discussed in detail. However, from the thick-
ness evaluations for the system PDIC+but-2-yne-l,4-diol,
which vary from 9.5 to 6.1 A per cycle, it can be seen that the
growth is not linear.
Also aliphatic polyureas have been fabricated by MLD using
only one diisocyanate, namely 1,4-diisocyanatobutane. ED,
7V-(2-aminoethyl)ethane-l,2-diamine, N,iV-bis(2-amino-
Polythiourea
The only MLD-grown polythiourea was fabricated by using
1,4-diisothiocyanatobenzene and ED as the precursors
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precursor B
reference
1 ,4-diisocyanatobenzene
o=c=nH^^n=c=o
1 ,4-diisocyanatobenzene
0=C=N^\ /^N=C=0
but-2-yne-1,4-diol
HO
terephthalic acid bis(2-hydroxyethyl) ester
[74]
[74]
cr
(Table 8). The deposition temperature in these experiments was
kept at 50 °C and the films were grown on silicon and silica
nanoparticles. The obtained growth rates were 1.9 and 2.8 A per
cycle, respectively. These values are modest when comparing to
the calculated chain length of 13.8 A [75].
Polyesters
Polyesters are commonly made with reactions between various
acids and alcohols. The only MLD polyester reported is poly-
ethylene terephthalate (PET), which is an industrially widely
used polymer. The precursors employed for the fabrication of
PET films were terephthaloyl dichloride, which is an acid chlo-
ride, and ethane- 1,2-diol (ethylene glycol, EG), a diol (Table 9).
The deposition temperature range investigated was 145-175 °C.
The films were grown on Si(100) substrates, cleaned ultrasoni-
cally in ethanol and water. The depositions were carried out on
both cleaned substrates and on substrates that were further func-
tionalized with amine groups using (3-aminopropyl)triethoxysi-
lane. The growth rates were significantly better with the func-
tionalized substrates. The highest growth rate, 3.3 A per cycle,
was obtained at 145 °C. The length of a PET molecule is 1 1 A,
so only partial layer coverage was achieved, though [76].
Polyimines
Terephthalaldehyde and 1,4-diaminobenzene have been used to
form polyimine thin films (Table 10) at room temperature
Table 9: Precursors used to deposit polyester thin films.
precursor A
precursor B
reference
terephthaloyl dichloride
ethane-1,2-diol
C> /=\ CI
HO^° H
[76]
[9,77,78]. The growth process was investigated by using
Au-coated glass substrates with a self-assembled monolayer of
11-amino-l-undecanethiol. According to the investigations by
Yoshimura et al. [78] the polymer wires grew in upward direc-
tions. Later experiments showed that that the first six cycles
yielded a GPC value of 10 A per cycle, after which the growth
rate started to decline [77]. Different combinations of
terephthalaldehyde, 1,4-diaminobenzene and ethanedihydrazide
have been used to fabricate quantum dots of varying lengths:
These thin films showed promise for sensitization in photo-
voltaic devices [79,80].
Hybrid inorganic-organic thin films
Since the first publications featuring inorganic-organic hybrid
thin films, the number of articles relating to this type of films
has been rapidly increasing. In the following subchapters the
inorganic-organic thin films deposited by using the combined
precursor A
precursor B
reference
1 ,4-diisothiocyanatobenzene
S=C=N^^^N=C=S
ethane-1,2-diamine
H2N ^NH 2
[75]
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Table 10: Precursors used to deposit pol
precursor A
precursor B
precursor C
references
terephthalaldehyde
KK
terephthalaldehyde
O /=\ H
1 ,4-diaminobenzene
1,4-diaminobenzene
ethanedihydrazide
,, O
kl ,NH 2
[9,77,78]
[79,80]
ALD and MLD technique are presented. The different thin films
produced are divided by the organic precursor employed. The
subchapters "Alcohols and phenols", "Acids" and "Amines"
contain all the different hybrid films made by using an organic
precursor with hydro xyl, carboxyl or nitrogen atom function-
ality, respectively. It should be noted that some of precursors
under "Alcohols and phenols" have also another functional
group, such as 4-aminophenol with both -OH and -NH2
groups. (2S)-2-Aminopentanedioic acid, an amino acid with
carboxylic acid functionality, is considered under "Acids".
"Other organic precursors" consist of all the organic precursors
that do not fall clearly into the categories mentioned above. The
thin films based on 7-octenyltrichlorosilane are presented in
their own subchapter at the end of this chapter.
Alcohols and phenols
Most of the inorganic-organic hybrid thin films reported so far
have been deposited by using alcohols or phenols as the organic
precursor (Table 11). When metal precursors are combined with
alcohols or phenols, the resultant material is often called
"metalcone" [27]. Most of the research has been focused on the
aluminum-based alucones [12,13,15,16,38,39,81-97], but also
zincones [13,33,38,39,46,51,52,56,94,95,98-103], titanicones
[53,55,95,104,105], and zircones [54,87] have been studied.
The schematic structure of ideally growing TMA+ethane- 1 ,2-
diol, DEZ+benzene-l,4-diol and TMA+oxiran-2-ylmethanol
hybrid films are shown in Figure 6 as examples of these types
of films.
Ethane- 1,2-diol (ethylene glycol, EG) is the simplest diol used
to fabricate inorganic-organic hybrid materials by ALD/MLD.
The use of EG together with TMA was first reported by
Dameron et al. [12]. The thin films were grown in the tempera-
ture range of 85-175 °C. The GPC value for the films varied
from 4.0 to 0.4 A per cycle, decreasing with increasing deposi-
tion temperature. The TMA+EG hybrids were unstable in
cursors used to deposit inorganic-organic hybrid thin films,
organic precursor inorganic precursor
ethane-1,2-diol
HO^-° H
propane-1,2,3-triol
AI(CH 3 ) 3
Zn(C 2 H 5 )2
T1CI4
Zr(C 4 H 9 0) 4
AI(CH 3 ) 3
Zn(C 2 H 5 ) 2
TiCI 4
references
[12,15,16,81-89,91,92,95,96]
[51,52,98,101]
[53,104,105]
[54,87]
[95]
[95]
[53,95,105]
hexa-2,4-diyne-1 ,6-diol
HO^
OH
Zn(C 2 H 5 ) 2
TiCI 4
[46,56]
[55]
1116
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11: Alcohol and phenol precursors used to deposit inorganic-organic hybrid thin films, (continued)
benzene-1,4-diol
.OH
HO
AI(CH 3 ) 3
Zn(C 2 H 5 ) 2
[13,94]
[33,95,99-103]
benzene-1,3,5-triol
OH
HO
OH
AI(CH 3 ) 3
[13]
oxiran-2-ylmethanol
U^OH
4-aminophenol
H 2 N-
OH
AI(CH 3 ) 3
Zn(C 2 H 5 ) 2
Zn(C 2 H 5 ) 2
TiCI 4
[38,39,90,93,97]
[38,39]
[36,103,106]
[37]
8-quinolinol
tris(2-hydroxyethyl)amine
HO^ N ^OH
s
OH
AI(CH 3 ) 3
Zn(C 2 H 5 ) 2
TiCU
AI(CH 3 ) 3
[13,35]
[13,35]
[13,35]
[107]
2-aminoethanol + furan-2,5-dione
,NH 2 +
HO
O^ x O ^O
AI(CH 3 ) 3
[16,48,84,108,109]
2-aminoethanol + propanedioyl dichloride
O O
^NH 2 + ju
HO'
cr
-ci
TiCI 4
[49]
ambient conditions, but capping with Si02 improved the
stability. Mechanical studies of the TMA+EG hybrids showed
that the material is brittle, with a toughness of about
0.17 MPa-m 0 5 [81]. The brittleness was also observed with
nanointendation measurements, giving an elastic modulus of
about 37 GPa and a Berkovich hardness of about 0.47 GPa [84].
However, when the TMA+EG hybrid with an additional H2O
pulse was employed as an interlayer between ALD-grown
AI2O3 and a Teflon substrate, it was noticed that the stress
caused by the difference in the coefficient of thermal expansion
between the coating and the substrate was significantly reduced,
preventing the cracking of the AI2O3 coating [96].
The first zincones were fabricated by combining EG with DEZ.
This type of hybrid was reported by Peng et al. [51] and Yoon
et al. [52]. The former group deposited the thin films in the
temperature range of 100-170 °C. The maximum GPC value,
about 0.57 A per cycle, was achieved for the film deposited at
120 °C, while the lowest GPC, about 0.39 A per cycle, was
observed for the films deposited at 165 °C. However, the
1117
Beilstein J. Nanotechnol. 2014, 5, 1104-1136.
Figure 6: Schematic illustration of ALD/MLD inorganic-organic hybrid thin films deposited by using (a) TMA with ethane-1 ,2-diol, (b) DEZ with
benzene-1 ,4-diol, and (c) TMA with oxiran-2-ylmethanol.
DEZ+EG hybrids are unstable in ambient air and the thickness
measurements were carried out on reacted films with a reduced
thickness. The films by Yoon et al. [52] were deposited at
temperatures between 90 and 170 °C. Unlike with the deposi-
tions by Peng et al. [51], the thickness of the films decreased
with increasing temperature. A growth rate of 0.7 A per cycle
was measured for films deposited at 130 °C. Both groups
reported that although the DEZ+EG hybrid reacts with water in
ambient air, the films remained stable after an initial quick reac-
tion. Recently Liu et al. [101] grew DEZ+EG hybrid films at
150 °C. They observed that the GPC varied strongly depending
on the number of deposition cycles, from 0.39 to 0.86 A per
cycle for 500 and 2000 cycles, respectively. This phenomenon
was speculated to be due to the double reactions occurring
during the growth. Also the GPC is far below the length of a
Zn-EG unit, which was estimated to be around 6.9 A. Thermal
conductivity of the DEZ+EG hybrid was measured to be around
0.22-0.23 W/(m-K), with little variation in the values when
thicker samples were analyzed. The volumetric heat capacity
was about 2.5 J/(cm 3 -K) [101].
Ethylene glycol has also been used with TiCl4 to deposit titani-
cone films [53,104,105]. Deposition temperatures for the
TiCl 4 +EG films varied from 90 to 135 °C. The GPC first
decreased gradually from 4.6 A per cycle at 90 °C to 4.1 A per
cycle at 125 °C, after which there was a larger drop to 1 .5 A per
cycle at 135 °C. The TiCLi+EG hybrid thin films were unstable:
The thickness diminished by 15% over five days and after
25 days the total reduction was 20%. The elastic modulus and
hardness values measured by using nanointendation were
extremely low, i.e., about 8 GPa and about 0.25 GPa, respect-
ively [53].
Zircone thin films have been fabricated by using EG together
with zirconium tert-butoxide (ZTB) [54,87]. The depositions
were carried out in the temperature range of 105-195 °C. The
GPC decreased with increasing deposition temperature, from
1.6 to 0.3 A per cycle. The films were stable in ambient air: A
decrease in thickness of only ca. 3% was observed after expo-
sure in ambient air for one month [54].
Titanicone films have been also made by using propane- 1,2,3-
triol (glycerol, GL) together with TiCl4. The deposition
temperature regime used for these films was between 130 and
210 °C. The GPC value at 130 °C was 2.8 A per cycle and
dropped with increasing temperature gradually to 2.1 A per
cycle at 210 °C. A small increase of film thickness was
observed when thin films were exposed to air. The elastic
modulus and hardness were both higher for the GL titanicones
than those based on EG, with values of about 30.6 GPa and
about 2.6 GPa, respectively. The films were also more ther-
mally stable than the EG-based counterparts, probably due to
crosslinking in the structure [53].
Hexa-2,4-diyne-l,6-diol (HDD) has been combined together
with DEZ and TiCl4 to form cross-linked polydiacetylene struc-
tures. These films were polymerized by using UV irradiation
after the DEZ/TiC^ and HDD pulses. Deposition temperatures
of 100-150 °C, and 100 °C were used for the Zn- and
Ti-containing hybrid films, respectively. The calculated ideal
layer thickness for both types of hybrids was approximately
6 A. The GPC of the DEZ+HDD films was close to the ideal,
i.e., 5.2 A per cycle, whereas the measured GPC for the
TiCl4+HDD system reached full mono layer coverage, i.e., 6 A
per cycle. Investigation of the electrical properties of the
DEZ+HDD thin films revealed the films had an excellent field
effect mobility (>1.3 cm 2 -V~'-s). When TEM was used to
observe structures obtained with 50 cycles of TiC>2 and one
cycle of TiCl4+HDD, individual nanolayers from each material
could be seen [55,56].
1118
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Benzene- 1,4-diol (hydroquinone, HQ), a phenol with two -OH
groups in a para position, and TMA has been used to deposit
films in the temperature range of 150-400 °C. The GPC was
quite constant regardless of the deposition temperature used,
around 3.5 A per cycle. When the films were stored in ambient
air for one week, a decrease of 25% in film thickness was
measured [13]. Zn-containing HQ films have also been fabri-
cated with DEZ. The DEZ+HQ films were grown at tempera-
tures between 130 and 170 °C, yielding GPC of 1.6 A per cycle
at 150 °C. The stability of these films was not discussed [33]. In
another study, a GPC of 2.7 A per cycle at the deposition
temperature of 150 °C was obtained. Although considerably
higher than when comparing to GPC obtained earlier, it is still
far from the ideal growth, which was estimated to be around
8.4 A per cycle. Thermal conductivity of the DEZ+HQ films
was considerable higher than those of the DEZ+EG films, i.e.,
about 0.32-0.38 W/(m-K): The difference was attributed to a
more vertical growth in case of the hybrid using the aromatic
precursor when compared to the one using the linear precursor.
Also the volumetric heat capacity was higher with the DEZ+HQ
than with DEZ+EG, i.e., about 3.1 J/(cm 3 -K) [101].
Benzene- 1, 3, 5-triol, an aromatic compound with three -OH
groups, has been used to fabricate hybrid thin films together
with TMA. The depositions were carried out in temperature
range of 175-400 °C. The GPC was about 5 A per cycle regard-
less of the used deposition temperature. When stored in ambient
air for one week, an increase of 9% in film thickness was
measured [13].
There have been several studies featuring inorganic-organic
hybrid materials deposited by using TMA and oxiran-2-
ylmethanol (glycidol, GLY) [38,39,90,93,97]. The GLY mole-
cule is heterobifunctional with both hydroxy and epoxy func-
tionalities. Heterobifunctionality is believed to reduce the
number of double reactions. During the deposition an epoxy
ring-opening reaction takes place: The reaction is strongly
dependent on the use of strong Lewis acids such as TMA. The
first detailed studies of the growth of TMA+GLY thin films
were published by Gong et al. [38] and Lee et al. [39] The
temperature regimes used by the two groups were 90-150 °C
and 100-175 °C, respectively. The GPC values reported varied
greatly. Gong et al. [38] reported GPC values which decreased
with increasing deposition temperature, the values varying from
24 (which is considerably higher than the calculated chain
length) to 6 A per cycle, whereas Lee et al. [39] reported a
growth rate of only 1.3 A per cycle at 125 °C. Both groups
reported that the TMA+GLY thin film growth is sensitive to
reaction conditions. As the paper by Lee et al. [39] was the one
published a little later, they speculated that the difference in
their GPC values when compared to those reported by Gong et
al. [38] could be due to CVD-type growth regime at lower
deposition temperatures and with short purging times. Gong et
al. [38] observed that the TMA+GLY films were relatively
stable in ambient air: A 168 hour exposure resulted in the
absorption of some OH~ according to FTIR data, but no change
in thickness was detected. Both groups performed experiments
also with DEZ+GLY systems, but no decent growth was
observed for these films. It was concluded that DEZ as a weaker
Lewis acid is not able to sufficiently catalyze the reaction
required for the film growth to proceed [38,39].
The 4-aminophenol (AP) molecule is heterobifunctional
consisting of a benzene ring with both -OH and -NH 2 groups.
It has been investigated together with the inorganic precursor
DEZ. The DEZ+AP hybrid thin film depositions were carried
out at temperatures between 140 and 330 °C. Rather constant
GPC of about 1.1 A per cycle was obtained at deposition
temperatures of 140-200 °C, after which the GPC started to
decrease with increasing temperature. The resultant films were
stable when stored in ambient air when the humidity level was
low [36]. Recently AP was used together with TiCl4, deposited
at temperatures between 120 and 220 °C. The obtained GPC,
which was 10-11 A per cycle in the deposition temperature
range of 140-160 °C, was close to the value calculated from the
bond lengths, 9.1 A per cycle. The films were relatively stable:
Less than 10% decrease in film thickness was observed when
stored in ambient air for 800 h [37].
Aromatic 8-quinolinol has been used together with TMA, DEZ
and TiCLj [13,35]. The deposition temperature ranged from 85
to 200 °C. The maximum growth rates were about 4, 6.5 and
7.5 A per cycle for the A1-, Zn-, and Ti-containing hybrids, res-
pectively, and were obtained at the lowest deposition tempera-
ture used for each system. The growth rate gradually decreased
with increasing temperature for all processes and no growth was
observed in the films deposited at 200 °C. Significant photo lu-
minescent activity was observed for the Al- and Zn-based
hybrids whereas in the Ti-containing films a detectable but only
small activity was perceived [35].
Tris(2-hydroxyethyl)amine, a tertiary amine, has been
combined together with TMA to form hybrid thin films. The
depositions were performed at 150 °C. The GPC was 1.3 A per
cycle and the films were unstable in ambient air [107].
The use of more than two precursors can mitigate the proba-
bility of double reactions. Yoon et al. [48] were the first to
make inorganic-organic hybrid thin films by using three
different precursors, namely TMA, 2-aminoethanol and furan-
2,5-dione. From the two organic precursors, 2-aminoethanol is
heterobifunctional while furan-2,5-dione is a ring-opening reac-
1119
BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
tant. The depositions were done in the temperature range of
90-170 °C. The GPC was 24 A per cycle at 90 °C and 4.0 A per
cycle at 170 °C. The films were not stable: It was observed that
the films react with water mostly within the first 10 min of
exposure to ambient air [48]. In later studies it was deduced that
the growth process of the films is governed by the TMA diffu-
sion into and out of the forming film, making the growth
process strongly dependent on the TMA dose and purge times
used. Also experiments carried out at 1 30 °C showed that the
self-limiting growth could be only observed when the films
remained thin [108]. According to nanointendation measure-
ments conducted for the TMA+2-aminoethanol+furan-2,5-dione
films deposited at 90 °C, the elastic modulus and Berkovich
hardness were about 13 GPa and about 0.27 GPa, respectively
[84].
Later, Chen et al. [49] also utilized three different precursors to
fabricate hybrid thin films, i.e., TiCl/i, 2-aminoethanol and
propanedioyl dichloride. Unlike with the TMA+2-amino-
ethanol+furan-2,5-dione system, the growth sequence of these
films did not consist of repeated introduction of the precursors
in three stages, but in four: One full deposition cycle consisted
of supplying TiCU, followed by 2-aminoethanol, then propane-
dioyl dichloride and finally a repeated pulsing of 2-amino-
ethanol. The heterobifunctionality of the 2-aminoethanol mole-
cule was expected to improve reaction selectivity of the process.
A GPC value of 6 A per cycle was achieved at deposition
temperature of 100 °C when grown on carbon nanocoils.
Acids
Although the deposition of inorganic-organic hybrid thin films
by using carboxylic acids as organic precursors was first
mentioned by Nilsen et al. [13] already in 2008 (Table 12), the
later articles published by the same group discuss the topic in
greater detail [32,57,110], separately for saturated [57] and
unsaturated [110] linear acids, as well as aromatic acids [32],
when combined with TMA. The saturated linear acids were
observed to form mostly bidentate complexes, and the unsatu-
rated linear acids formed either bidentate or bridging
complexes, whereas for the aromatic acids the complex type
varied depending on the acid. Only one carboxylic acid has
been used in combination with an inorganic precursor different
from TMA, namely benzene- 1,4-dicarboxy lie acid with zinc
acetate [111]. Representatives for the possible film structures
are depicted in Figure 7.
In the works reported for the combination, TMA+saturated
linear acid, the chain length of the acid varies from two carbon
atoms for the ethanedioic acid to ten carbon atoms for the
decanedioic acid. Accordingly, the GPC values for the different
systems also vary greatly (Figure 8), from 43 A per cycle
achieved with the heptanedioic acid with seven carbon atoms to
Table 12: Carboxylic acid precursors used to deposit inorganic-organic hybrid thin films
organic precursor
inorganic precursor
references
ethanedioic acid
HO .0
AI(CH 3 ) 3
[13,57]
propanedioic acid
O O
HO'^^OH
AI(CH 3 ) 3
[13,57]
butanedioic acid
HC V^OH
AI(CH 3 ) 3
[57]
O
pentanedioic acid
O O
AI(CH 3 ) 3
[13,57,112]
propane-1 ,2,3-tricarboxylic acid
0 H °Y° OH
AI(CH 3 ) 3
[112]
HO^^^^O
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BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
heptanedioic acid
O O
HO - v v v - 0H
octanedioic acid
O
HO^
decanedioic acid
HO
(E)-butenedioic acid
O
"OH
AI(CH 3 ) 3
AI(CH 3 ) 3
AI(CH 3 ) 3
AI(CH 3 ) 3
[13,57]
[13,57]
[13,57]
[13,110]
(Z)-butenedioic acid
O O
HO "OH
(1E)-prop-1-ene-1,2,3-tricarboxylic acid
O
HO
HO
AI(CH 3 ) 3
AI(CH 3 ) 3
[13,110]
[110]
(2E,4E)-hexa-2,4-dienedioic acid
O
Ha - " " " ' OH
AI(CH 3 ) 3
[13,110]
benzoic acid
/ = \ OH
benzene-1 ,2-dicarboxylic acid
O
OH
OH
AI(CH 3 ) 3
AI(CH 3 ) 3
[32]
[13,32]
benzene-1 ,3-dicarboxylic acid
O O
HO
OH
benzene-1 ,4-dicarboxylic acid
HO /=\ 9
AI(CH 3 ) 3
AI(CH 3 ) 3
Zn(CH 3 C0 2 ) 2
[13,32]
[13,32]
[111]
1121
Beilstein J. Nanotechnol. 2014, 5, 1104-1136.
Table 12: Carboxylic acid precursors used to deposit inorganic-organic hybrid thin films, (continued)
benzene-1 ,3,5-tricarboxylic acid
O O
HO
OH
O' °H
benzene-1 ,2,4,5-tetracarboxcylic acid
O O
HO
HO
OH
OH
O O
(2S)-2-aminopentanedioic acid
O O
HO
OH
NH,
AI(CH 3 ) 3
AI(CH 3 ) 3
AI(CH 3 ) 3
[13,32]
[13,32]
[112]
(a) (b) (C) <
Figure 7: Schematic illustration of ALD/MLD inorganic-organic hybrid
thin films deposited using TMA together with (a) propanedioic acid
(bidentate complex), (b) (E)-butenedioic acid (bridging complex), and
(c) benzene-1 ,3,5-tricarboxylic acid (unbidentate complex).
about 1 A per cycle for the decanedioic acid. From Figure 8, all
the other systems but the one using heptanedioic acid and
propane- 1, 2, 3-tricarboxylic acid show a decrease in GPC with
increasing deposition temperature. For the heptanedioic acid the
GPC is highly dependent on the deposition temperature, and the
maximum value attained at 1 62 °C is even four times higher
than the chain length of the acid precursor, a detail which could
not be explained by the authors. The GPC of propane- 1,2,3-tri-
carboxylic acid remained constant in the temperature range
investigated. From Figure 8, the maximum GPC values for the
acids containing 2-5 carbon atoms were of the same order of
magnitude, whereas higher values were obtained for the acid
systems with seven and eight carbon atoms and lower values for
the decanedioic acid with ten carbons [57,112].
From Figure 8, when disregarding the heptanedioic acid+TMA
hybrid, the GPCs for the unsaturated acids employed were
higher when compared to those of saturated acids. The
maximum GPCs varied between 7.8 and about 15 A per cycle
obtained for (Z)-butenedioic acid and (l£)-prop-l-ene,l,2,3-tri-
carboxylic acid systems. For all systems, the GPC decreased
with increasing deposition temperature [110,112].
The aromatic carboxylic acids used to fabricate hybrid films
together with TMA vary from each other regarding the number
of carboxyl groups attached to the aromatic ring and how the
groups are positioned. Benzoic acid has only one carboxyl
group, which may explain why no decent growth for these films
were observed. From dicarboxylic acids all the possible group
positions were investigated, i.e., benzene- 1 ,2-dicarboxylic acid,
benzene- 1,3-dicarboxy lie acid and benzene- 1,4-dicarboxy lie
acid. The maximum GPC value for all these three acids, as seen
from Figure 8, was in the range of 8 ± 1 A per cycle. The
aromatic carboxylic acid containing three carboxylic groups,
benzene-1, 3, 5-tricarboxylic acid, produced the highest GPC
values from the aromatic acids: The maximum GPC was as high
as 13.4 A per cycle. The maximum GPC of benzoic-1, 2,4,5-
tetracarboxylic acid was considerably lower, 5.6 A per cycle.
As can be seen from Figure 8, for all aromatic acid
systems the GPC decreases with increasing deposition tempera-
ture [32].
All the carboxylic acid+TMA hybrid films were stable: No
changes were observed for the saturated and unsaturated acid
systems when stored in ambient air for one year, whereas the
1122
Beilstein J. Nanotechnol. 2014, 5, 1104-1136.
20
15
saturated
unsaturated
10 -
A.
A
100
100
o o
o
♦
□ ♦
X
o
A t
o
f
150
200 250 300
Deposition temperature (C"
150
200 250 300
Deposition temperature (C°)
Oethanedioic acid (2C)
50 r
Apropanedioic acid (3C)
45 -
• butanedioic acid (4C)
40
■ pentanedioic acid (5C)
35 -
♦ propane-1 ,2,3-tricarboxylic
acid (6C)
□ octanedioic acid (8C)
30
25 ■
xdecanedioic acid (10C)
20
0(E)-butenedioic acid (4C)
15
A(Z)-butenedioic acid (4C)
10 -
■ (1 E)-prop-1-ene-
1 ,2,3,tricarboxylic acid (6C)
5
0
Aheptanedioic acid (7C)
350 400 • (2E,4E)-hexa-2,4-dienedioic
acid (6C)
X benzoic acid
■ benzene-1,2-dicarboxylic acid
♦ benzene-1,3-dicarboxylic acid
Abenzene-1,4-dicarboxylic acid
• benzene-1,3.5-tricarboxylic acid
Obenzene-1,2,4,5-tetracarboxylic acid
350 400
Figure 8: Growth per cycle values for inorganic-organic hybrid films deposited by using TMA with different carboxylic acids [32,57,1 10,1 12].
stability of aromatic carboxylic acids was observed for one
week [32,57,110,112].
Salmi et al. [11 1] used benzene- 1,4-dicarboxy lie acid together
with zinc acetate to deposit metal-organic framework (MOF)
thin films. The films were deposited at 225-350 °C, and the
GPC decreased with increasing deposition temperature, the
highest value being 6.5 A per cycle. No growth was observed
when the deposition was carried out at 350 °C. The as-deposited
films were amorphous, but films deposited below 300 °C
crystallized when kept at 60% humidity at room temperature.
According to the time-of-flight elastic recoil detection analysis
and FTIR experiments conducted for the crystallized films, the
composition was close to that of MOF-5.
Klepper et al. [112] used (25)-2-aminopentanedioic acid, an
amino acid with a carboxylic acid functional group, together
with TMA. The films were grown in the temperature range of
200-350 °C. The GPC values decreased with increasing deposi-
tion temperature. The highest GPC obtained was 20 A per
cycle, which is considerably higher than the estimated chain
length, which was 10 A. However, according to the XRD
measurements, there was a peak around 4.5° in the 9-29 pattern,
which corresponds to a d value of 19.9 A, indicating an Al-Al
sheet distance, which is close to the maximum GPC. The group
concluded that a sheet-like structure with an interplanary sheet
distance of 20 A forms during the deposition of the film. Also
these films were stable: No changes were observed visually or
in XRR and FTIR measurements.
Amines
Two diamines, 1,4-diaminobenzene and ODA, have been used
to fabricate inorganic-organic hybrid thin films by ALD/MLD
(Table 13). The 1,4-diaminobenzene, i.e., an amine equivalent
of HQ, was used together with TMA. The depositions were
carried out in the temperature range of 200-400 °C. The GPC
decreased with increasing deposition temperature, varying
between about 1 and about 2 A per cycle. The obtained films
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BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
is.
organic precursor
inorganic precursor
references
1 ,4-diaminobenzene
4,4'-oxydianiline
H 2 N 0 "^J^" N H2
AI(CH 3 ) 3
TiCI 4
[34]
[14,103,113]
were unstable: An increase of ca. 30% in film thickness was
observed when the films were kept in ambient air for two weeks
[34].
of 14 s. The GPC of these films also increased with increasing
temperature, from 0.8 A per cycle at 160 °C to 1.6 A per cycle
at 310 °C [113].
The amine precursor ODA has been used together with TiCLj to
deposit thin films at deposition temperatures varying from 160
to 490 °C. GPC of these films increased with increasing deposi-
tion temperature, from 0.3 A per cycle to 1.1 A per cycle with
an ODA-pulse length of 3 s. The TiCl4 films were stable in
ambient air when deposited at 250 °C and above. Wet-etching
testing revealed that when treated with toluene, acetone,
methanol, 1 M acetic acid, or water, no significant change in
film thickness was observed [14]. However, as the ODA-pulse
length of 3 s was not enough for the fully-saturated growth, in a
later study TiCl4+ODA films with longer ODA pulse lengths
were deposited. It was observed that an ODA pulse length of
12-16 s led to saturated growth. Depositions at temperatures
from 160 to 310 °C were carried out with an ODA pulse length
Other organic precursors
Only three organic precursors have been used that cannot be
included in the alcohol/phenol, acid or amine categories,
namely ethanetetracarbonitrile, which is, like the name indi-
cates, a nitrile, 2-oxepanone, a cyclic ester, and 1,2-
bis[(diamethylamino)dimethylsilyl]ethane, which is a hybrid
material in itself (Table 14).
Ethanetetracarbonitrile has been used together with vanadium
hexacarbonyl V(CO)6 to produce the only vanadium-containing
hybrid thin film deposited until now. The depositions were
carried out at room temperature. A GPC of about 9.8 A per
cycle was achieved. The film was a room-temperature magnet,
with a large coercive field (ca. 80 Oe at 5 and 300 K).
organic precursor
ethanetetracarbonitrile
inorganic/counter precursor
reference
1,2-bis[(dimethylamino)dimethylsilyl]ethane
V(CO) 6
AI(CH 3 ) 3
0 3
[114]
[47]
[115]
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BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
According to the experiments, the hybrid has a high T c and/or
high thermal stability when compared to corresponding CVD-
made films. It was concluded that the material holds promise
for next-generation spin-related applications [114].
Gong et al. [47] used 2-oxepanone and TMA to make hybrid
thin films in which the TMA as a Lewis acid catalyzes a ring-
opening reaction. The films were deposited at 60-120 °C. The
GPC value decreased with increasing temperature being 0.75 A
per cycle at 60 °C GPC but only 0.08 A per cycle at 120 °C.
The films were stable; no change in film thickness was
observed when stored in ambient air for 30 days.
The hybrid film fabricated by Zhou and Bent [115] differs in
some aspects from the other hybrids made up till now. It is the
only carbosiloxane film made so far and the precursor used is
an inorganic-organic hybrid material in itself, namely 1,2-
bis[(dimethylamino)dimethylsilyl]ethane. Both O3 and H2O
were tried as the second precursor, but H 2 0 did not work as
well as O3. It was speculated that double reactions hinder the
growth when using H2O, whereas when O3 was used new
surface groups were generated, improving the overall growth
process. GPC of 0.2 A per cycle was achieved at the deposition
temperature of 110 °C for the l,2-bis[(dimethylamino)di-
methylsiryl]ethane+C>3 system. The films were extremely stable,
withstanding well wet etching treatments of tetramethyl-
ammonium hydroxide, HC1 and acetone as well as vacuum
annealing: When annealed at 300 °C, no change in thickness
was observed, 400 °C resulted in a thickness change of 6%, and
after annealing at 600 °C, a thickness loss of 13 % was
measured.
7-octenyltrichlorosilane based thin films
ALD/MLD has been used to deposit several different inor-
ganic-organic hybrid films featuring 7-octenyltrichlorosilane
(7-OTS) as one of the precursors (Table 15). The first article
where 7-OTS was employed was published already in 2007
[10]. However, 7-OTS in itself cannot be said to be a fully
organic material as it contains silicon. Also 7-OTS was pulsed
to the reactor together with water as a catalyst, not alone as in a
conventional ALD/MLD process. The key stages for all these
7-OTS hybrids during one cycle were the same: First 7-OTS
and water were introduced, followed by an ozone treatment.
The third stage consisted of metal precursor connection to the
forming chain, followed by reaction with water. Schematic
presentation of the forming chain is shown in Figure 9. The
metal precursors used to make these type of hybrid thin films
include Ti(OCH(CH 3 ) 2 ) 4 [10,40], TMA [46], Zr(C 4 H 9 0) 4 [43]
and DEZ [44]. For all but TMA thin films the GPC was around
10-11 A per cycle. The maximum growth rate for the
Table 15: Inorganic-organic hybrid thin film systems based on 7-octenyltrichlor<
precursors of organic layer precursors of inorganic layer(s) references
7-octenyltrichlorosilane
CI
Cl-Si-CI
H 2 0
O3 Ti(OCH(CH 3 ) 2 ) 4 H 2 Q
10,40
7-octenyltrichlorosilane
CI
Cl-Si-CI
H 2 0
O3
Ti(OCH(CH 3 ) 2 ) 4
AI(CH 3 ) 3
H 2 Q
41,42
7-octenyltrichlorosilane
CI
Cl-Si-CI
H 2 0
o 3
Zr(C 4 H 9 0) 4
H 2 Q
43
7-octenyltrichlorosilane
CI
Cl-Si-CI
H 2 0
o 3
Zn(C 2 H 5 ) 2
H 2 Q
44
7-octenyltrichlorosilane
CI
Cl-Si-CI
H 2 0
0 3
AI(CH 3 ) 3
H 2 Q
45,46
1125
Beilstein J. Nanotechnol. 2014, 5, 1104-1136.
7-OTS+TMA film was only 6 A per cycle. There are some
contradictions regarding the reported growth rates for a full
monolayer covering. According to the earlier articles an ideal
monolayer would have a length of 12 A [10,41] whereas a more
recent one states that the full monolayer coverage would lead to
a GPC value of 25 A per cycle [40].
Figure 9: Schematic illustration of an ALD/MLD inorganic-organic
hybrid thin film deposited using 7-octenyltrichlorosilane and
Ti(OCH(CH 3 ) 2 ) 4 .
The 7-OTS+Ti(OCH(CH 3 ) 2 ) 4 containing films have been
grown in a temperature range of 150-200 °C. When annealed,
these films were stable up to 450 °C. When the electrical prop-
erties of the films were investigated, the experiments showed
that the current density decreased with increased film thickness.
When the films were less than 6.7 nm thick, direct tunneling
was observed while the 99 nm thick films had a good gate
leakage resistance. The dielectric constant for the films was
around 17 at 1 MHz. It was concluded that the system
7-OTS+Ti(OCH(CH 3 ) 2 ) 4 is a good candidate for high-quality
gate-insulating films on flexible substrates [10]. In later experi-
ments rapid water permeation speed was observed for this type
of film [40].
Although the first article mentioning the use of 7-OTS together
with TMA was published already in 2008 [41], a more thor-
ough investigation of the growth process was carried out only
recently [45]. The GPC decreased with increasing temperature,
from about 6 A per cycle at 100 °C to 2.5 A per cycle at 200 °C
[45].
The Zr- and Zn-containing films were grown at 170 and 150 °C,
respectively. These studies focus more on the nanolaminates
fabricated with oxides and will be discussed in more detail later
[43,44].
Processes with no growth details
In this chapter ALD/MLD works about only thin films that
provide no precise details regarding the experiments are shortly
summarized. It should be noted that the organic precursors used
in the growth processes are not included in any of the tables.
The only Fe-containing hybrid films reported so far were grown
by Smirnov et al. [11,116] from FeCl3 and 2-propyn-l-ol
(propargyl alcohol, the simplest alcohol with an alkyne group).
The deposition temperature was 200 °C, but no other experi-
mental details were given. The focus was on the magnetic prop-
erties of the films: Uncompensated antiferromagnetism was
observed.
Nilsen et al. [13] mention the use of nona-l,9-diol (1,9-nonane-
diol) together with TMA at 200 °C, but besides a QCM figure
there is no further information regarding the system. In the
same article TiCl4 and ED were used to fabricate hybrid thin
films. Only a QCM graph acquired at deposition temperature of
75 °C and a cautioning to use long pulse lengths were given.
They also combined 4-aminobenzoic acid with TiCl4 hoping
that the carboxylic group would enhance the air stability of the
hybrid film. No information regarding the depositions of these
films was given but a QCM graph obtained at the deposition
temperature of 200 °C.
The deposition strategy consisting of four stages by Chen et al.
[49] has been also used to fabricate Al-containing hybrid films
from TMA. The organic precursors were the same as with the
Ti-containing counterparts, i.e., aminoethanol and propanedioyl
dichloride.
Abdulagatov et al. [105] conducted pyro lysis studies on several
hybrid films grown by using EG, GL, HQ and 2,3,5,6-tetrafluo-
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BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
robenzene-l,4-diol (tetrafluorohydroquinone) as the organic
precursors: Al+GL, Al+EG, Al+HQ, Al+2,3,5,6-tetrafluoroben-
zene-l,4-diol, Zn+GL, Zn+HQ, Zr+EG, Hf+EG and Mn+EG
were deposited at 150 °C [105]. Preliminary FTIR studies
measuring 2,2-dimethoxy-l,6-diaza-2-silacyclo-
oxetane+ethylene carbonate and TMA+3-buten-l-ol containing
systems have been shortly mentioned [60].
In his review article, George [26] mentions polydimethyl-
siloxane films deposited by using H2O together with bis(di-
methylamino)dimethylsilane, 1 ,3-dichlorotetramethyldi-
siloxane and dimethylmethoxychlorosilane. The GPC of the
films was negligible after 15 deposition cycles, so depositions
consisting of four pulsing steps, i.e., TMA+H 2 0+dimethyl-
methoxychlorosilane+H20, at 200 °C were tried as well.
Although this process yielded linear growth with GPC of 0.9 A
per cycle, the silicon content was too low. Hybrid films made
by using TMA and l,4-diazabicyclo[2.2.2]octane (triethylenedi-
amine) are also shortly mentioned.
Tetrakis(dimethylamido)hafnium and EG have been combined
to form hafnicones. Linear growth was observed at the deposi-
tion temperature of 145 °C. GPC decreased from 1.2 A per
cycle at 105 °C to 0.4 A per cycle at 205 °C. The films were
said to be very stable [28].
Layer-engineered and nanostructured ALD/
MLD films
Making various nanostructure and multilayer architectures by
ALD and MLD is inherently easy due to the self-limiting cyclic
growth process, which allows excellent control over the layer
thickness and enables the conformal growth of the films. The
ALD/MLD techniques have been successfully utilized to coat
various nano structures, such as nanoparticles, nanowires and
carbon nanotubes. Both post-deposition annealing and wet-
etching processes have been employed to remove the organic
part of the inorganic-organic hybrid material, leaving porous
oxide backbones. By adding a second material to the deposition
process, it is also possible to make thin-film mixtures, superlat-
tices and nanolaminates with tailored properties. In the
following subchapters depositions of such nanostructured and
layer-engineered ALD/MLD materials are discussed.
Fabrication of porous and nanostructured materials
Hybrid ALD/MLD processes based on TMA and EG precur-
sors have been used to coat nanoparticles and nanowires: TiC>2
nanoparticles have been coated for photoactivity passivation
[85], silica nanoparticles to fabricate AI2O3 films with
controlled pore sizes (Figure 10a) [15,82], and CuO nanowires
to obtain hollow A1 2 C>3 nanostructures [83]. Hybrid TMA+EG
coatings have also been utilized to fabricate A^CVzeolite
composite membranes. In the latter example the organic
constituent was removed by means of a post-deposition oxi-
dation to form microporous AI2O3 on and between zeolite crys-
tals. The membranes showed promise for H2 separation [89].
Liang et al. [98] deposited DEZ+EG coatings on titania
nanoparticles. The DEZ+EG hybrid was observed to reduce the
photoactivity of the nanoparticles. When the films were
annealed in air, porous ZnO was formed. However, the number
of pores was rather low. Moreover, the size of the pores fluctu-
ated widely.
Porous TiC>2 films have been obtained both by post-deposition
annealing and by treating TiCl4+EG films with UV light
[53,104]. After the post-deposition annealing the films were
observed to have both amorphous and crystalline states and ex-
hibit high photocatalytic activity. In a more recent paper by
Abdulagatov et al. [105], GL was used together with TiCl4 to
deposit thin films which then were annealed to form porous
structures. For the annealed films a significantly reduced sheet
resistance was achieved, being as low as 2.2 * 10 2 Q. for films
annealed at 800 °C; resistivity of the same films was 0.19 £l-cm.
These films are anticipated to have some potential in applica-
tions related to electrochemical reactions and electrochemical
energy storage [105].
Gong et al. [38] deposited hybrid TMA+GLY coatings on
electro-spun polyvinyl alcohol fiber mats, and subsequently
annealed the samples at 400 °C for 48 h to remove the organic
part. Conformality was verified by transmission electron
microscopy, see Figure 10b. The pores formed were found to be
mostly 5 A micropores, with some mesopores [38]. Lee et al.
[39] investigated the effect of the annealing temperature for
similar types of TMA+GLY films and found that annealing at
350-500 °C removed the organic part of the films leaving
A1 2 0 3 .
Hybrid TMA+GLY thin films have also been used together
with ALD-grown ZnO and TiC>2 to coat and protect electro-
spun polyamide-6 (PA-6) nanofibers [90,93]. When TiC>2 was
used alone, the process caused fiber degradation. It was
revealed that whereas a layer of ZnO did not offer sufficient
protection from the following TMA+H2O treatment, when a
layer TMA+GLY was deposited after ZnO, it prevented further
degradation of the fibers when a Ti02 coating was applied on
the top [90].
Recently, Brown et al. [95] coated carbon nanotubes with
several different metalcone materials, including the TMA+EG,
TMA+GL, TiCl 4 +GL, and DEZ+GL systems, see Figure 10c.
The measurement of mechanical properties revealed that the
1127
Beilstein J. Nanotechnol. 2014, 5, 1104-1136.
Figure 10: Electron microscope images of (a) 250 nm silica particles coated with a 25 nm thick layer of TMA+EG (reprinted with permission from [15],
Copyright (2009) The Royal Society of Chemistry), (b) TMA+GLY film deposited on electro-spun polyvinyl alcohol fibers and annealed at 400 °C for
48 h (reprinted with permission from [38], Copyright (201 1) American Chemical Society), and (c) metalcone coated carbon nanotubes (reprinted with
permission from [95], Copyright (2013) American Chemical Society).
MLD coatings caused a significant reduction in failure strain, a
modest improvement in ultimate tensile strength, and a signifi-
cant improvement in Young's modulus. The greatest failure
strain, about 3.9%, was achieved for TMA+GL coated
nanotubes. The highest average Young's modulus and the ulti-
mate tensile strength were found for TMA+EG coated samples,
with values of about 7 GPa and about 100 MPa, respectively.
Young's modulus for the uncoated carbon nanotube was
510 MPa, 2.2 GPa for 10 nm thick TMA+GL coated samples,
and about 9 GPa for a composite coating consisting of 5 nm
TMA+GL and 5 nm A1 2 0 3 .
Hybrid TMA+GLY films have also been grown on hydrophobic
polydimethylsiloxane (PDMS) silicone exposed to sequential
vapor infiltration of TMA+water. The aim of the study was to
produce PDMS with a hydrophilic surface. When pure AI2O3
was used to coat the PDMS, a hydrophilic surface was obtained.
However, after storing for 48 hours in ambient air the coating
became more hydrophobic, losing the desired wetting character-
istics. The TMA+GLY coating produced also a hydrophilic
surface, which retained the hydrophilicity for more than two
weeks in ambient air. However, the most hydrophilic and stable
coatings were obtained by fabricating structures consisting of
100 cycles of A1 2 0 3 and TMA+GLY [97].
Liang et al. [109] deposited TM A+tris(2-hydroxy-
ethyl)amine+furan-2,5-dione thin films at 150 °C on 500 nm
spherical silica particles. Carbon was removed from the films
by soaking them in water for one day or by a 1 h oxidation in
air at 400 °C. The resultant nanopores were 0.6-0.8 nm (and
some 17 nm pores) and 0.8 nm in size, respectively.
Dry-etching with oxygen of TMA+EG and TMA+tris(2-
hydroxyethyl)amine+furan-2,5-dione hybrids and wet-etching
of TMA+tris(2-hydroxyethyl)amine+furan-2,5-dione with
various solvents has also been studied. It was observed that HC1
could be used to etch the thin film in a controlled manner,
which makes of TMA+tris(2-hydroxyethyl)amine+furan-2,5-
dione hybrid films promising materials for MEMS/NEMS
applications [16].
Films from TiCl4, tris(2-hydroxyethyl)amine and propanedioyl
dichloride have been fabricated on suspended CuO nanowires
and carbon nanocoils by using a four-deposition-stage ap-
proach. The films were annealed at 600 °C in 5% H 2 /Ar for 2 h
after the deposition. Excellent photocatalytic activity was
observed for the nanoporous TiC^/carbon nanocoil structures.
Nanoporous AI2O3 structures were also obtained using TMA as
the inorganic precursor [49].
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Fabrication of thin-film mixtures, superlattices and
nanolaminates
The ALD/MLD thin-film mixtures, superlattices and nanolami-
nates are all made by using at least two different materials, for
example a purely organic material and an oxide, by varying the
number of deposition cycles of each material. The distinction
between the various types of such layer-engineered materials is
not completely unambiguous. In principle, a mixture is formed
when the number of deposition cycles of each material is kept
relatively low; then, if no full monolayer coverage is achieved
with each material, the materials do not form separate layers but
instead a homogenous mixture. On the other hand, if the growth
happens in a well-controlled manner, repeated cycles may lead
to a superlattice structure. The nanolaminates are formed by
using larger number of deposition cycles, so that at least one of
the materials achieves nanometer scale thickness and the ma-
terials form individual layers in the structure.
Only three articles published so far feature multilayer struc-
tures that contain purely organic nanoscale layers [62,70,72],
Salmi et al. [62] fabricated Ta205/polyimide nanolaminate
structures consisting of nanoscale inorganic Ta 2 Os and organic
polyimide layers. Tantalum ethoxide and water were used to
deposit Ta 2 05, and PMDA and hexane-l,6-diamine for the
polyimide deposition. All nanolaminates were constructed from
five bilayers, with three different constructions: 10 nm Ta 2 Os +
10 nm polyimide (shown in Figure 11), 15 nm Ta 2 Os + 5 nm
polyimide, and 5 nm Ta 2 Os + 15 nm polyimide. The study
focused on dielectric and mechanical properties of these
nanolaminates. It was observed that the permittivity of the
nanolaminates increased with the Ta 2 Os content, but not in a
linear manner. The trend was similar for the refractive index,
but in a more linear way. It was also seen that the insulating
Ta 2 O s Polyimide
Substrate
Figure 11: Field emission scanning electron microscopy image of a
nanolaminate fabricated using five bilayers of 10 nm Ta20s and 10 nm
polyimide (reprinted with permission from [62], Copyright (2009)
WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).
properties of the parent materials could be improved through
the fabrication of nanolaminate structures. Nanointendation
measurements revealed that the softness, elasticity and plas-
ticity of the films increased with increasing polyimide content.
Loscutoff et al. [70] deposited purely organic nanolaminates at
room temperature consisting of 30-50 nm thick PDIC+ED and
PDIC+2-[(2-aminoethyl)sulfanyl]ethanamine layers. The
growth of the latter material was not discussed in detail in the
article, though. Depth profiles by a sputter technique were
investigated on two different nanolaminates: The first consisted
of two layers of 40 nm PDIC+ED, with a 50 nm thick PDIC+2-
[(2-aminoethyl)sulfanyl]ethanamine layer in between, the
second of two 30 nm thick PDIC+2-[(2-aminoethyl)sulfan-
yl]ethanamine layers with a 40 nm thick PDIC+ED layer
between them. Although a small unexpected sulfur signal indi-
cated the presence of some unreacted sites, it could be
concluded that the composition of the nanolaminates agreed
well with the expected values.
As part of their study on potential photoresist materials, Zhou et
al. [72] also investigated nanolaminates of purely organic
constituents, i.e., PDIC+ED and PDIC+2,2-(propane-2,2-
diylbis(oxy))diethanamine. Organic films of the former type are
stable in HC1, whereas of the latter type are not. Three-layer
structures were deposited to investigate the stability of the
nanolaminates in HC1: First 3, 6 or 9 cycles of PDIC+ED were
deposited, followed by 3 or 6 cycles of PDIC+2,2-(propane-2,2-
diylbis(oxy))diethanamine, finished with 3 cycles of PDIC+ED
on the top. According to the thickness measurements, only the
bottom layer remained after the acid treatment, indicating that
the cleaving occurred at the positions of acid-labile groups.
Also nanolaminates consisting of 3 cycles of PDIC+2,2-
(propane-2,2-diylbis(oxy))diethanamine and 8 or 15 cycles of
PDIC+ED on top were deposited. These films were soaked with
triphenylsulfonium triflate, UV radiated, baked and developed.
No significant changes in thickness were observed and it was
concluded that the top layer behaves as a photoacid generator
barrier.
Quite many inorganic-organic hybrid compositions have been
utilized to fabricate homogeneous thin-film mixtures, superlat-
tice structures and nanolaminates. Lee et al. [86,87] fabricated
mixtures consisting of TMA+EG alucone and Al 2 03 at 135 °C,
and varied the oxide:hybrid cycle ratio from 1:3 to 6:1 to accu-
rately control the density, refractive index, elastic modulus and
hardness of the films. The values obtained varied between those
for pure alucone and pure Al 2 03, for density from 1.6 to
3.0 g/cm 3 , for refractive index from 1.45 to 1.64, for elastic
modulus from 20 to 200 GPa, and for hardness from 1 to
13 GPa.
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BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
The stability and optical properties of mixtures of alucone (from
TMA+EG) and AI2O3 have been also studied and compared to
those of pure alucone films. The depositions were carried out on
Si(100) substrates at 150 °C. It was observed that when the
sample was kept in air the thickness and refractive index of pure
alucone decreased by about 20% and about 1 .4%, respectively,
over the first three days, after which there were no further
changes. When the alucone films were kept in a desiccator,
there were no significant changes in thickness or refractive
index. It was also revealed that capping a 100 nm thick alucone
layer with a 20 nm thick AI2O3 layer improved the stability, as
did the fabrication of a nanolaminate structure consisting of five
bilayers of 20 nm of alucone and 10 nm of AI2O3. When homo-
geneous alucone:Al 2 C>3 mixtures were investigated, the mix-
ture with a ratio of 5:1 lost 12-17% of its initial thickness in
open air, whereas the 5:5 mixture was stable, and the thickness
of the 1:1 mixture was reduced by 4%. The change in refractive
index for the 5:1 mixture was larger than for the pure alucone
film. Measurements performed on annealed films showed that
the refractive index could be tuned by the speed of the
annealing: Slow annealing resulted in a film with more pores
and less collapse in the film. When heated, the 1:1 mixture
showed numerous cracks while the 5:1 mixture showed better
resistance to the heat treatment. When the 5:1 mixture was
heated with 10 °C/h, the refractive index dropped from the
initial 1.52 to 1.34 while the thickness decrease was about 28%.
In case of the 5:5 mixture heating did not destroy the film, but
the refractive index decreased to 1.44 [92].
Zhou et al. [34] deposited mixtures consisting of TMA+1,4-
diaminobenzene hybrid and AI2O3 with the hybrid:oxide ratios
of 1:1, 1:2, 1:3 and 1:4. The 1:4 mixture showed improved
stability in ambient air: The film thickness decreased less than
5% when kept at ambient air for several tens of weeks. The
mixtures showed excellent electrical insulating properties. A
current density of 2.3 x 10~ 8 A-cm -2 at an electric field of
1 MV-cm"', and a dielectric constant of about 6.2 were
measured for the 1:4 mixture. The 1:4 mixture showed clear
charge trapping behavior, but not good enough to be used as a
charge trap layer for a charge trap flash memory.
Miller et al. [81] investigated the mechanical properties of
AI2O3/TMA+EG/AI2O3 nanolaminates grown at 155 °C on
polyethylene naphtalate substrates. The layer thicknesses were
10/3/10 nm, 25/15/25 nm (shown in Figure 12), 25/3/25 nm,
and 25/192/25 nm. The nanolaminates exhibited reduced crit-
ical strains at fracture when compared to pure components. This
was attributed to the low toughness of the TMA+EG alucone
[81]. According to the microcantilever-facilitated curvature
studies done later the curvature for the pure TMA+EG hybrid
evolved when the films were stored in ambient air for two
weeks. Investigation on 25/192/25 nm thick A1 2 0 3 /TMA+EG/
AI2O3 nanolaminates showed that the topmost AI2O3 layer
stabilized the structure, possibly by shielding the underlying
TMA+EG layer from moisture [84].
Nanolaminates fabricated using TMA+EG and AI2O3 were also
investigated by Vaha-Nissi et al. [88]. The depositions were
carried out at 100 °C on biopolymer (biaxially oriented poly-
lactic acid) substrates, and the samples were characterized for
their oxygen transmission rate (OTR) and water vapor transmis-
sion rate (WVTR). It was shown that the five-layer nanolami-
nates investigated, i.e., AI2O3/TMA+EG/AI2O3/TMA+EG/
AI2O3, worked essentially better than pure AI2O3 films as gas-
barrier coatings on biopolymer materials. Purely inorganic coat-
ings are somewhat brittle (as clearly evidenced from SEM
images revealing large cracks for pure AI2O3 films on top of
flexible biopolymer substrates), and straining them leads to
defects and deteriorated gas-barrier properties. Apparently the
intervening hybrid layers improve the flexibility of the coating.
For the laminates the defect concentration was found to be
considerably smaller compared to AI2O3 films after mechanical
straining. The best barrier properties were achieved for the five-
layer laminate deposited with 50 cycles of both components.
The laminates were also found to be stable in air which was not
the case for a thin AI2O3 layer alone on the biopolymer sub-
strate.
The WVTR of TMA+EG and AI2O3 coatings was investigated
also by Park et al. [91]. The films were deposited at 85 °C on
polyethylene naphthlate substrates. The WVTR values
measured at 85 °C and 85% relative humidity for alucone,
AI2O3 and Al 2 C>3/alucone nanolaminate coatings were ca. 1.1,
0.037 and 0.021 g/(m 2 -day), respectively. The improved WVTR
value of the nanolaminate coating when compared to the pure
alucone and oxide layers alone was attributed to a synergy
Pt/Au
ALDAI 2 0 3
MLD Alucone
ALDAI2O3
PEN
100 nm
Figure 12: A nanolaminate coating consisting of AI2O3 and TMA+EG
alucone layers with targeted thicknesses of 25 and 15 nm, respective-
ly, on a polyethylene naphtalate (PEN) substrate (reprinted with
permission from [81], Copyright (2009) American Institute of Physics).
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BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
effect: As the alucone layer reacts readily with water, it reduces
the diffusion speed as well as increases the diffusion paths of
the water vapor.
The Zn-based hybrid DEZ+HQ has been combined with ZnO to
form mixtures which were anticipated to show enhanced elec-
trical conductivity due to electron band overlap between the
Jt-electrons of the HQ ring and ZnO [98,99]. The pure ZnO film
had a conductivity of 14 S/cm in ambient light, whereas the
pure hybrid film was nonconductive. The 1:1, 1:3 and 2:2
mixtures of ZnO:DEZ+HQ had higher electrical conductance
than ZnO, i.e., 1 16, 40 and 170 S/cm, respectively, whereas the
1:5 and 5:5 mixtures exhibited lower conductance than ZnO,
i.e., 6 and 13 S/cm. respectively. The elastic modulus varied
depending on the precise composition of the mixtures, being
about 30 GPa for a pure DEZ+HQ hybrid film, about 50 GPa
for a 1:5 mixture, about 120 GPa for a 1:1 mixture, and about
150 GPa for a pure ZnO film. Also the hardness of the films
was affected by the moiety concentration, being about 1,4,5
and 13 GPa for the same films, respectively. Pure DEZ+HQ, the
1 : 1 mixture and the pure ZnO were all highly transparent in
visible spectrum [99].
Superlattice structures have been successfully fabricated where
nondoped or Al-doped ZnO layers of nanometer-scale alternate
with extremely thin organic (hydroquinone) layers. The films
were deposited with the DEZ+H 2 0, TMA+H 2 0 and DEZ+HQ
processes at 220 °C, with the oxide:hybrid deposition cycle
ratio varying from 199:1 to 1:1 [94,102,103]. FTIR and XRR
studies confirmed the incorporation of organic layers and the
formation of superlattice structures, respectively. When heated
for one hour up to 700 °C, no changes in XRR patterns of the
(Zn,Al)0:HQ films were observed until 450 °C, after which
coarsening of the film started to inflict noise. The superlattice-
originated XRR peaks were still present when annealed up to
650 °C. No visual change in the films was observed until heat-
treated at 700 °C. Apparently the ZnO layers protect the under-
lying organic layers from decomposition at elevated tempera-
tures. The eventual goal of the study was to suppress lattice
thermal conductivity and/or enhance the Seebeck coefficient of
ZnO films through the introduction of intervening organic
layers such that the superlattice films could show overall
enhanced thermoelectric characteristics, i.e., concomitant large
Seebeck coefficient, high electrical conductivity and low
thermal conductivity. Preliminary characterization of the
(Zn,Al)0:HQ superlattice films confirmed that the films indeed
showed promise as thermoelectric materials [94]. More recently
it was demonstrated that similar superlattice structures
consisting of thicker layers of ZnO combined with individual
organic layers could be made not only with HQ but also with an
AP or ODA layer [103]. The ZnO:hybrid ratio varied between
199:1 and 39:1, and the superlattice structure was confirmed by
XRR. Resistivity and Seebeck coefficient measurements
showed an increase in carrier concentration for small concentra-
tions of organic layer, whereas at higher concentrations a large
reduction in carrier concentration was observed.
Liu et al. [101] fabricated oxide-hybrid mixtures by using the
same DEZ+H2O and DEZ+HQ processes as described above
for the ZnO:HQ superlattices, and determined the thermal
conductivity for the 1:1 mixture at about 0.13-0.15 W/(m-K).
This value is much lower than what was measured by the same
group for pure DEZ+HQ hybrid and discussed earlier in this
review. It was suggested, that when employing structurally
vastly different oxide and hybrid constituents in the material
fabrication, the ZnO flakes and hybrid chains scatter efficiently
phonons, resulting in a reduced thermal conductivity. The
volumetric heat capacity for the 1:1 mixture was about
2.9 J/(cm 3 -K), being only slightly less than what was reported
for the DEZ+HQ system.
The DEZ+AP and DEZ+H2O processes have been combined to
form mixtures and nanolaminates. The crystallinity and density
of the mixtures were varied by the number of hybrid and oxide
cycles during the depositions. Although the hybrid containing
films were unstable in air, which made the AFM measurements
somewhat inaccurate, adding amorphous hybrid to crystalline
ZnO had a smoothing effect on the samples. The nanolaminates
which had a minimum of 3 nm thick ZnO layer on top were
stable in ambient air and had a rather constant RMS roughness
of 1-1.4 nm. According to the nanointendation measurements
the DEZ+AP hybrid was soft, with a contact modulus of a
typical polymer. Although ZnO is also soft, it is still harder and
stiffer than the pure hybrid. Adding ZnO to the hybrid was
observed to have little effect up to 1:1 hybrid:oxide ratio. From
the nanointendation measurements performed on nanolami-
nates it was concluded that the thicker the ZnO layer is, the
more it enhances the hardness of the film and a thin ZnO layer
can actually reduce film strength for some unknown reason.
Wet-etching tests showed that adding ZnO does now improve
the chemical stability of the mixtures. However, acetone treat-
ment was observed to remove only the organic part of the film,
leaving a porous oxide backbone [106].
Mixtures of Ti02 and TiCl4+ODA have been deposited with
oxide:hybrid ratios from 1:1 to 10:1. A good control over the
RMS roughness value and refractive index was achieved by
varying the mixture composition. Also the density, reduced
modulus, hardness and crystallinity of the material systemati-
cally depended on the oxide:hybrid ratio. Wet-etching tests
carried out with several solvents indicated that the mixtures
were chemically extremely stable [113].
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For ZrC>2:ZTB+EG mixtures in which the oxide:hybrid ratio
was varied from 1 :3 to 3: 1 [54,87], it was found that the refrac-
tive index and elastic modulus changed continuously from
values for pure ZTB+EG in ~ 1 .63 and E~21 GPa) to those of
pure ZrC>2 (n ~ 1.86 and E ~ 97 GPa). Also density and hard-
ness varied according to the oxide:hybrid ratio.
Superlattice structures consisting of TiC>2 and T1CI4+HDD were
successfully deposited at 100 °C by repetition of 50 cycles of
Ti02 and one cycle of T1CI4+HDD. TEM images showed indi-
vidual oxide and hybrid nanolayers. The films remained stable
when annealed up to 400 °C [55].
As 7-OTS is a heterobifunctional material, for which the
terminal C=C group is required to be converted to a carboxylic
group by ozone treatment during the growth process, the 7-OTS
hybrid materials grow in extremely well controlled layer-by-
layer manner. Hybrid films based on 7-OTS have been used in
several studies to fabricate various superlattice and nanolami-
nate structures. Four bilayers consisting of 1.1 nm thick
7-OTS+Ti(OCH(CH 3 ) 2 ) 4 hybrid and 2 nm thick Ti0 2 were
used to fabricate nanolaminates. The nanolaminate was then
sandwiched between Al metal wires to study the electrical prop-
erties of the hybrid-oxide material. The constructed device had
a large endurance and a long retention time, which demon-
strated the potential application of the nanolaminates as
nonvolatile memory materials [10].
Investigations on multilayered structures of
7-OTS+Ti(OCH(CH 3 ) 2 ) 4 and Ti0 2 deposited on PEN indi-
cated that the Ti02 blocked water permeation. The WVTR
experiments suggested that the lag time for the structures was
extended due to the tortuous path effect and water accumula-
tion in the organic layers. For a structure consisting of five
bilayers a WVTR value of 7.0 x 10~ 4 g/(m 2 -day) during a lag
time of 155 h at 60 °C and a relative humidity 85% was
obtained [40].
Superlattices with hybrid-Al203-hybrid-Ti02 structures with
various mixing ratios have been fabricated using
7-OTS+Ti(OCH(CH 3 ) 2 ) 4 and 7-OTS+TMA. The formed struc-
tures when annealed were stable up to about 500 °C. The coat-
ings showed good flexibility, were mechanically stable, and had
various unique electrical properties. Organic pentacene thin-
film transistors fabricated by using the superlattices on flexible
plastic substrate had a drain current of 1.5 uA, a field effect
mobility of 0.54 cm 2 /(V-s), and an inverter voltage gain
-dF 0Ut /dKj n ~ 4.5 when operated at a voltage of -2 V [41].
A non-volatile flash memory thin-film transistor was made
using 7-OTS+Ti(OCH(CH 3 ) 2 ) 4 and 7-OTS+TMA layers
between ZnO and pentacene. The device showed promising
non-volatile memory effects when operated at low voltages
[42]. Organic pentacene thin- film transistors were also fabri-
cated by using 7-OTS+ZTB and Zr02- A maximum field effect
mobility of 0.63 cm 2 /(V-s) was measured, when operating at
-1 V with an on/off current ratio of about 10 3 [43].
Nanolaminates consisting of 7-OTS+DEZ and ZnO layers have
also been deposited. The thin-film transistors made by using the
nanolaminate had a high field effect mobility of 7 cm 2 /(V-s),
when operating at 3 V with an on/off current ratio of 10 6 and
with a threshold voltage of 0.6 V. It was also concluded that the
7-OTS+DEZ provides structural flexibility in the superlattice
[44].
Han et al. [46] fabricated floating-gate nonvolatile memory
transistors from two types of hybrid layers: Al-containing
hybrid layers were deposited by using 7-OTS, water, O3 and
TMA as precursors, whereas DEZ and HDD were used for the
Zn-containing layers. Capacitor memory devices constructed by
using Al-containing hybrid as blocking and tunneling layers
with ZnO:Cu charge trap layer sandwiched between them
(Figure 13), had a large memory window of 14.1 V operated at
±15 V. The same structure was then used together with a
Zn-containing hybrid as a semiconducting layer to form
nonvolatile memory transistors which operated in voltage range
of-1 to 3 V. The high writing/erasing (+8 V/— 12 V) current
ratio of 10 3 obtained with the device indicated that the tested
construction showed promise for memory electronics applica-
tions.
Figure 13: An HRTEM image of a capacitor memory device fabricated
by using Al-containing hybrid (marked as AlOx-SAOL) as blocking and
tunneling layers with a ZnO:Cu charge trap layer in between (reprinted
with permission from [46], Copyright (2012) The Royal Society of
Chemistry).
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BeilsteinJ. Nanotechnol. 2014, 5, 1104-1136.
Conclusion
The application perspectives of the original ALD thin-film tech-
nology have become considerably wider through the introduc-
tion of purely organic moieties as building units in the chem-
ical atomic-scale controlled deposition process. Now by taking
advantage of the MLD technique we are not only able to materi-
alize in a molecular layer-by-layer manner high-quality thin
films of various commercially attractive organic polymers but
also of inorganic-organic hybrid materials, potentially
combining the best attributes of the two entirely different
chemistries.
The organic polymers made up till now by means of the MLD
technique include various amides, imides, imide-amides, ureas,
urethanes, esters and imines, while in the case of the ALD/
MLD-grown inorganic-organic hybrid thin films the metal
species variety covers the elements Al, Zn, Ti, Zr, Hf, V, Si, Fe
and Mn. Although a rapidly increasing number of different
precursors have already been exploited, the field is nothing but
just approaching its emergent stage. Nevertheless essentially
ideal MLD and ALD/MLD processes have already been devel-
oped for several precursor combinations such that the film
growth rates achieved well correspond to the values calculated
on the bases of the expected lengths for straight polymer chains.
Examples of such ideally behaving processes are the purely
organic hexanedioyl dichloride+hexane-l,6-diamine and
heptane- 1 ,7-diamine+nonanedioyl dichloride systems, and the
hybrid diethylzinc+hexa-2,4-diyne-l,6-diol, TiCi4+hexa-2,4-
diyne-l,6-diol and TiCl4+4-aminophenol systems.
The layer-by- layer manner in which the films are grown in both
ALD and MLD, provides us yet another powerful means of
fine-tuning the film properties by depositing on-demand
designed thin-film mixtures, superstructures and nanolaminates.
Optical and mechanical properties, surface roughness and
degree of crystallinity have been successfully tuned by mixing
different deposition cycles, whereas control over the chemical
stability, electrical and gas-barrier properties and electrical and
thermal conductivities has been achieved by constructing well-
defined superlattice and nanolaminate structures. Post-deposi-
tion treatments of films containing organic moieties have also
proven to further expand the application range of the ALD/
MLD fabricated hybrid thin films as such treatments enable,
e.g., to produce porous coatings.
The work on the ALD/MLD grown organic and
inorganic-organic thin films is still in its beginning phase.
Deeper studies are definitely required to shed light even on the
precise growth mechanisms of these fundamentally new types
of thin films, hopefully giving us better insight to select new
well-behaving precursor pairs. As the huge potential of the
hybrid films has been recognized, the number of articles
featuring properties related to specific applications keeps rising.
Recently, it was for example demonstrated that periodically
repeating organic layers embedded in thicker inorganic layers
can efficiently block heat conduction. This result is highly
promising in the field of thermoelectrics. Tunable refractive
index should on the other hand be extremely important for
optical applications. As another example, nanolaminate struc-
tures from oxides and hybrids improve the gas barrier prop-
erties of the protective coatings. Especially noteworthy are also
the porous structures, which could be used in optics, electronics
and catalysis, to name just a few examples. In short, the
prospects of the ALD/MLD fabricated films are excellent.
Acknowledgements
The present work has received funding from the European
Research Council under the European Union's Seventh Frame-
work Programme (FP/2007-2013)/ERC Advanced Grant Agree-
ment (No. 339478), and also from the Academy of Finland (No.
255562). Some of the early papers referred to here were found
via the Virtual project on the history of ALD (VPHA).
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